Basically they're when you do some kind of training and write down the results. The idea is that you measure your progress against where you have been by looking back at where you were.
So a simple pushup training log would be like this:
"Today I did 62 pushups without resting, a new personal best for me."
That's my understanding, anyway. Correct me if I'm wrong, guys.
EDIT - also note that the regular "training" section still exists for generic discussions about training-related stuff.