It is a way of saying there is not much else significant enough to be said; that is, the person has given you all the important information.

Note that by saying "pretty much it", it does not mean they have told you everything that could be said upon the matter, but as good as having done so.

Example, I ask you what you did today; you will tell me of all the highlight moments of the day, but you will not of course bore me with every small detail such as "at 11:05am, I sneezed"... So, once you have done telling me of all the noteworthy events and actions of the day, you end by saying "and that is pretty much it".

it means "that's all".

It means that there is nothing more to add to the story.