What is a media kit for bloggers?

It is like a resume for your blog / your blogger profile.
It should be a very brief summary of your reach, achievements, brands worked with, service/products offered height your blog.
The client/brand should be able to get convinced that you are a good person to collaborate with by seeing your media kit, even without seeing or talking to you in person.

A blogger media kit is a document used to outline important statistics and information about your blog to help potential brands and advertisers make a decision as to whether they want to work with you or not. Think of it as a snapshot of your blog's greatest achievements.