To be featherbrained is to be empty-headed or silly. Your ditzy friend who's always giggling might be a little featherbrained.
If you're featherbrained, you're not too smart or serious — in fact, you're probably foolish and a little simple. It isn't a very complimentary thing to call someone featherbrained, since being described this way is always at least a little insulting. The implication is that a featherbrained person's head is full of something insubstantial — as light and fluffy as feathers — rather than a brain.