To have it in you means that you have abilities and skills which you do not usually use and which only show themselves in a difficult situation. There are a number of ways you could use this phrase but one example would be, if speaking to a friend.......'You were brilliant yesterday, I didn't know you had it in you!! I hope this helps.

It means you did not believe someone was capable of doing something ... but then they did it. For example, someone passes a really difficult exam you expected them to fail.