According to the Collins Dictionary https://www.collinsdictionary.com/dictionary/english/fulcrum
A fulcrum is "the pivot about which a lever turns".
If you think of a teeter totter on a children's playground, the fulcrum is the base in the middle that the long wooden plank (or metal one depending on the playground) sits on. It is what allows the teeter totter to move up and down.
For our catapult, the fulcrum will be a stack of popsicle sticks, or you could try something different like an eraser, or really whatever you want. I suggest trying the popsicle sticks first, and then trying different things after to see which allows for the longest shots!