The baffle is the mesh that keeps the down from shifting between the compartments.

I've heard (or read) people refer to the compartments as baffles, but it's not really correct. I've also heard the sewn-through seams of jackets or lightweight bags referred to as baffles, but again, not correct.

Winter-weight bags are more likely to have the side block baffle.

It's also worth knowing that while most bags use vertical baffles, some use slanted baffles, which could be confusing when trying to feel them from the outside.