If you went to the lecture you woulda heard somebody have a speil about it. Its like some free voluntary thing where you meet people and discuss stuff and do activities with ACCG100.
Its pretty much like another resource place and a place to get help with ACCG stuff. All the guys who are helping you in the sessions have done ACCG100 before and are pretty much all D or HD students.
Its good for revision and very handy before the exams. And if you get lost in ACCG its the best place to come for help and ask any questions or anything at all.
I did it last year. Made a couple of good friends.