To nail it
means to do something perfectly or achieve success in a particular task or activity. It means to do something really well or perfectly.
Examples:
1-  After rehearsing his proposal speech, he nailed it, and she said yes!
2-  I thought I nailed it by attempting to impress my friends with my dance moves at the party, but I ended up tripping over my own feet and knocking over the snack table instead.