Can you tell me what is a mandolin?

The answer to your question is:  Yes I can.

Even though you haven't asked for the description, I'm already here
so I might as well go through it:

A mandolin is a small stringed instrument.  The strings are tuned like a
violin or the upper 4 strings on a guitar.  There's a pair of two strings close
together for each pitch, and it's usually played by wiggling a pick between
the pair, to keep the note going.