A drop-in session would usually be a session where students can come and go as they please, if they feel they need a little extra help with something or have some questions to ask. It does depend on the specific plan but you can also likely just drop in and get some work done independently, but knowing there’s help if you need it.
I would assume that it’s not compulsory, and it’s certainly not compulsory for the entire session, but it’s more on a here-when-you-need-it basis. However, I would strongly recommended popping in to the first one to get a feel of the expectations and try to go for a little while every week. It’s likely to be extremely useful and will benefit your overall understanding of the subject.