This applies probably much more broadly than to DiRT 3 only, but what do all the pace notes my co-driver provides mean? (Left|Right) [1-6]|"Hairpin" is easy enough to understand, along with the literal "keep left", "near house", but there are random numbers mixed in and other more abstract directions.
The general structure of pace notes is designed to inform the driver about the conditions of the course ahead in a shorthand fashion.
Most commonly they encode the distance to the next turn, the direction of the next turn, and the severity of the next turn. So a pace note might be "100, 5 Left" - which might mean "100 meters ahead is a severity 5 turn to the left." Higher severity in the DiRT 3 pacenotes indicates that the turn is shallow, and can be taken at a higher speed/gear.
Pace note structure and format varies, as it's essentially a form of rapid-fire communication between the driver and co-driver. Part of that is what's written on the page, and part of that is how the co-driver reads what's written.
If there's a specific term that's tripping you up, let me know and I'll try to find a specific reference for it.