Well, formally defined, reblogging is the mechanism in blogging which allows users to repost the content of another user's post with an indication that the source of the post is another user.
Wow, that's very wordy! Basically, when you reblog a post, you're posting content from another blog to your own blog and letting everyone know who you reblogged the post from. Let's say someone has posted the cutest puppy picture in the world. Here at newTumbl, you can reblog their post to your own blog to share the content wealth! The more reblogs a post has, the more popular a post will become!
It's also awesome when someone reblogs your post. Your great content would become more and more famous with every reblog, potentially turning you into a blogging superstar!
See Blog posts/Post basics/What is a reblog? in our Help Center for detailed information on how to reblog a post.