what does being "in form" mean?

A rider's form is propotional to the number of points scored over the last few stages. I don't know the exact calculation but a rider with a higher form score is one who has scored a higher number of fantasy points in their last stages.

It might not be applicable to the next stage though as you'd expect Contador to have a high form score coming out of the Vuelta but he's a) not at the TdU and b) not suited to flat stages.

Likewise having won the last stage Griepel's form figure will have increased but it doesn't mean he's necessarily suited to the next stage.

form is the average score over the last five stages the rider contested. so it can be a good indicator of how someone's riding, but be aware that some riders will still be carrying their form from many months back