Monkey see, monkey do is an idiom (Cast. “frase hecha, refrán”; Cat. “dita”). It means that we humans sometimes copy what others have, say or do because we think it’s better than what we have, say or do. For instance:
JOHN: Jason is always copying my T-shirts and trainers. I just can’t stand it!
PHIL: I know what you mean… He already copied my handwriting…
JOHN: Go figure… Monkey see, monkey do!
You can find more animal idioms here: http://www.englishclub.com/vocabulary/idioms-animal.htm