It means that you can put a 2bg stick and a 1gb stick into it, and it will be able to see and use all 3gb of ram, but this is the most it can see.
And that Apple says it can only support 2gb. This is because you have to have identical sticks to run the memory in dual-channel mode, and Apple likes dual-channel.
Basicly it means that if you need (or just want) to put more than 2gb of ram in your older macbook, you can, up to 3gb.