Everyone above has answered it for you. It's a positive statement meaning that you've done something really well. Other examples that have a similar meaning are: - You smashed it. - You owned it. These are slang, however, so I wouldn't advise using it during an exam :)

You "nailed it" means you completed a task successfully or got something right. It is a common English expression.

It's a slang expression for doing something really well. Be careful not to use the same expression with (him/her) instead of (it) - as the meaning changes to something else.

it means "you got it perfectly right", "did a great job" It seems to be becoming more and more common in recent years. I've heard it on TV shows like "masterchef" or some of the talent shows, when a contestant does really well, the judge might say "you nailed it".