The lid keeps the float piece from being pushed out of the main body by the rising pressure in the fermentor. It is needed to keep a water gap between the air outside and the atmosphere inside the fermentor.
That being said, just put the cap on and you'll be fine. And don't worry too much. For my first batch I managed to shove that little rubber grommet INTO the fermentor. Boy did I worry about an infection but my beer turned out great! Turns out this a lot harder to screw up than you'd think. Happy brewing!
Beer is proof that God loves us and wants us to be happy.
- Benjamin Franklin (and I don't care if this quote has been largely discredited/misquoted...I like it!)
Beer, if drank with moderation, softens the temper, cheers the spirit and promotes health.
- Thomas Jefferson