The story goes something like
If the land were privately owned, the owner would (clearly) run the optimum number, but because it is part of the commons, it is logical for people keep putting their sheep onto the land until it is totally grazed out.
An individual with some number of sheep is faced with the following choices. If the land has less than one sheep per acre, they will put theirs on (clearly). If the land already has more than one sheep per acre, it is clearly going to be grazed out, but they can't stop this anyway - so they may as well put their own sheep on the land and get some of the feed before this happens. Helping themselves to more reduces the total pool, but they still get more of it.
This provides an insight into the failures of communism.