(Not a Teacher)
If you were accepted by an exclusive college or club, someone might say, "Wow, you made it in!"
If you managed to infiltrate a well guarded area, you're mission lead could say, "Good, you've made it in."
If you got to your house just before a storm hit, you're wife might say, "Looks like you made it in just in time."
As billmcd said, there are a number of potential meanings, and it seems unlikely to me that you heard or saw this totally devoid of context. Even if it was scrawled on subway wall, where it was located could give a clue to the meaning.