The most common meaning of もったいない is "what a waste!" It often refers to wasting food or time etc.
ex: I leave my food unfinished and therefore food will be thrown away. Then you can say the ward.
"What a waste (of food)!"
I'm playing online games all day and do nothing besides that, you can say:
"What a waste of time!
Also, you can use it as in "too good".
ex: You bought an expensive gift for me, I would say:
「私にはもったいないです」, which means "that's too good for me". I think I don't deserve to receive the gift, that's why I say もったいない.