Ever see clips of a rally or someone playing a rally game and wonder what the co-driver is on about? Does he sound like he's having a stroke or speaking another language? What he's doing is serving as psychic powers, giving the driver knowledge of the road ahead before he gets there. He's not telling the driver what the road is doing right where they are or immediately ahead, but rather some distance ahead and often many turns ahead. This may seem like a distraction, and it does take some getting used to processing what he's telling you in addition to all the other things going on in and around your car, but his stream of information is invaluable to the driver because he can use it to anticipate how to take sections of the road before he's on them. Using the pace notes, the driver knows exactly how severe an upcoming turn is even before he can see it, and sometimes it's stuff obscured from view until it's too late.
But what does it all mean? Honestly, if one listens to enough of it and compares what they're seeing against what's being said, most people can probably deduce a lot of it. There is also, to my knowledge, no universal standard for pace notes. Some years back I was at the Southern Ohio Forest Rally and checking out cars and drivers before their first stage of the weekend, and some of them had their pace notes laying in the car on the seat or on the dashboard, and one thing I noticed is each team had their own version of notes. They didn't all note the same things and didn't note them the exact same way. The driver and co-driver are a team, and the most important thing is that they have a mutual understanding that makes sense to the pair of them. They each depend on the other, the driver relying on important input from the co-driver and the co-driver putting his life in the driver's hands.
I will try touching on what I feel are the most common and probably most understood notes. Remember that these aren't universal, and this isn't a complete list of every type of note that a team might use; far from it.
I feel like the most important notes for me are the turns - how sharp they are and which way they turn. Decades ago I had racing games that counted them 1 through 6, with 1 being most slight and 6 being most severe, but in more recent years the norm seems to be the reverse, with 6 being slight and 1 being sharp. There are more grades of turns, however. You may here "slight" which is, as the description implies, a slight turn barely off from straight, less severe than even a 6. At the other end there are turns sharper than 1, including "square" which is basically a 90-degree turn, "hairpin" which turns more than 90 degrees, and "acute" which is severely sharp.
So, from mildest turn to most severe, we have:
slight, 6, 5, 4, 3, 2, 1, 90, hairpin, acute
One way I've heard drivers look at it is 1 through 6 is what gear to take the turn at. This obviously isn't perfect, as there's so many variables including how many gears the car even has to begin with, but some use it as a general guideline. Of course, that wouldn't work with the opposite numbering with 1 being slight and 6 tight, but I've not heard that in use in a long time, myself. Another more basic alternative is to use terms like "easy," "medium," and "hard" or "sharp." Some games have this as optional notes for players more new to rally racing or that don't want to process as much information.
These turn measurements would be immediately followed by left or right, and sometimes with additional descriptions like "short," "long," "opens," or "tightens." So, if you have a "4 left opens" it starts out as a 4 to the left but gets milder as you go through it. Conversely, a "5 right tightens" starts as a 5 to the right but gets tighter further into it. "5 right tightens 2" would be a right turn that starts out milder, as a 5, but gets tighter down to a 2 later into the turn.
So what about straights? Well, we don't bother saying "straight." Pace notes do sometimes mention them, but not by use of the term "straight." Instead we get a measurement of how long the straight is, in yards or in meters. My co-driver might tell me "3 right opens, 120." This tells me that a moderate right turn opens into a straight, and that straight is 120 yards long. I don't think I've ever personally heard mention of really short straights, but rather we're probably typically looking at like 40 yards and up. I'm not saying nobody ever mentions anything shorter, and every team has their own notation, but I've never heard anything shorter. Either way, the main takeaway is that straights are given by their length, so the bigger the number the longer the straight.
A driver might be told to stay toward the inside or toward the outside of a turn, or to keep left or right along a straight, possibly to be positioned better for the next part of the road, or maybe to avoid a hazard. You may be told of a crest, a jump, or a big jump, with the former probably not resulting in leaving the ground. You may be warned of a "water hazard" or "water splash" which is just driving through water of noteworthy depth. Perhaps you may be warned of "off camber" or "bad camber" which means that the road leans toward the outside of the turn, which is the opposite of where we would prefer that it go. Objects near the road may be noted, including posts, trees, large rocks, bails, or houses. Crossing a bridge is typically worth mention. If there's a surface change, like from asphalt to gravel, or if it's winter and you go from cleared roads to icy patches, these will likely be noted.