No need to apologize. :)
Can't say I blame you for the confusion.
You should've seen us all stumbling around here this past July. LOL

What is this platform for?

For people to post what they consider to be good content (original content, in particular, is typically very successful).

What is the reason for upvote?

This platform isn't based off of "tipping", where you simply hand someone money for posting. It's based off upvotes. And the more Steem Power you have (whether that means actual money to purchase Steem, or just your time creating content and earning Steem Power), the more influence your vote has, meaning the more money the post you voted on will receive.

How do we know what to post?

Well, you don't, really. It's like fishing with a variety of baits. Original content - content that you yourself created - tends to be the best bait, in my opinion.

How do we know exactly what we cannot post?

See answer to previous question.

Don't get me wrong, there's certainly a learning curve, but that can be said of anything, really.
Let me know if you have any further questions. I'll be glad to answer them to the best of my ability.