Who Is A Witness? What Do They Do On Steemit?

Newcomers to this platform struggle to understand a lot of its terms. This includes me. I would appreciate it if someone Would explain, please, who a witness is. The legal usage is what keeps coming into my mind. And the religious group that bears the name "witnesses".

Who is a witness on steemit? What do they do? How can they help me? What may I do for them? I can't find anything about witnesses in the Welcome section.