When you say you 'smashed something' or someone 'smashed it', it is an informal expression meaning that you did something well or performed to a high standard. If you tell someone to 'smash it' you are saying words of encouragement or wishing them well.

1. My favourite team are already 3-0 up in the cup final; they are really smashing it at the moment!
2. I am studying hard because I really want to smash this history exam.
3. Good luck in your driving test, I am sure you are going to smash it!

1. Alice was very nervous about making the sales pitch, but in the end she absolutely smashed it. I think everyone was impressed.
2. You've done all your preparation and bought yourself a smart new suit. I really think you're going to smash that job interview.