I have never read the books, I've only seen the first two movies. I've wondered about what the imprinting part that jacob had to do and why it had to happen?

He didn't have to do it.

It's complicated to explain. It would be a lot easier for Du to read this page:
link

Imprinting is not something that Du have to do it just happens out of nowhere.It's a connection and a feeling that a wolf has that cannot be controlled. Some Wölfe imprint and some don't.
Jacob went to kill the baby when the imprinting happened. I suggest Du watch all the Filme to understand.

Basically, it happened in order to close off the Liebe dreieck fully, and to give Jacob a happy ending instead of having him mourning Bella for years. It also created peace between the Wölfe and the Cullens, and made it impossible for the Wölfe not to be fighting alongside them at the end.