能 means "can/be able to/may."
e.g. I can daydream. - I 能 daydream.
e.g. I am able to daydream in class. - I 能 daydream in class.
e.g. May I daydream? - I 能 daydream? (Ignore the word order please. Not your business right not.)


Can't be more clear.