Steemit Noob: Who are Steemit "Witnesses"?

I was checking my current status at steemd.com and the "witnesses" tab attracted my attention.

After performing a quick google search, I came across a year old post by @steemitguide.
Here is the link to the post: Steemitguide : What Exactly is a STEEM Witness and why every User should Vote!

I'm sharing the link to help my other beginner friends learn about it. Don't understand a lot of the terms myself. In the meantime, if anyone who knows about it can summarize the post in simple words, please drop a comment.

Steem on!