How do i vote on other people's posts?

I have written a few posts and have noticed some of my posts have been voted on.

How do i vote on other people's posts that i like?
Can i vote on as many posts as i like?
I have noticed when i write a post a little check box which says Upvote post, what does that mean??

If anyone could answer these questions in the reply section, i'd be very grateful.