1. To succeed at something in a particularly impressive way. Her presentation for the CEO went really well. She totally nailed it! A: "How was your interview today?" B: "Nailed it."
2. To do something perfectly or accurately. This phrase is often used humorously to describe failures. I had all the kids guess how many jellybeans were in the jar, and Michael nailed it—exactly 251 jellybeans! When I showed my friend a picture of the cake I was trying to bake, alongside my sad excuse for a replica, she started laughing and said, "Nailed it!"
See also: nail