If you're logged in, I think the ones at the top are the ones you're subscribed to. If you're not logged in, they might just be randomly selected popular ones? I honestly have no idea, I never look at it.

Everyone is automatically subscribed to the same default subreddits when they make an account. If you weren't subscribed to anything there would be nothing on your front page. The alternative to automatically doing this would be what Twitter does when you create an account, where they show you a bunch of users and ask you to follow a few. I prefer Reddit's approach of just signing you up for stuff that most people seem to like.

I would just ignore multireddits. They aren't particularly useful and you won't see them very much if you don't use them. In fact I've never even visited https://www.reddit.com/subreddits/, and I've had an account for 3 years. I don't even know how you got there from the front page.

No idea why only some are in bold, I guess the mods of each subreddit had the option of including bold text when they wrote the summary, and most decided not to.

Yes, https://www.reddit.com/r/pics/ is a subreddit. It is a single place where you can submit pictures. It is somewhat similar to a chan-like area in that /r/Games is where you go to talk about games, /r/news is where you talk about news, etc. Why can't you find answers by looking at the screen? I mean it says right on the side: "A place to share photographs and pictures." There is an about page (https://www.reddit.com/about) and a help page (https://www.reddit.com/wiki/index) that explains subreddits, but I think most people tend to figure it out on their own.

Yeah, /r/ means a subreddit, /u/ means a user. I don't really know why you're concerned with the URL though, you can get around easily by clicking links and ignoring URLs, like with most websites.

"ALL" is not all posts in /r/pics, it's all posts in all of Reddit. As you can see in the top right: "/r/all displays content from all of reddit, including subreddits you aren't subscribed to. Some subreddits have chosen to exclude themselves from /r/all." It probably looked the same as /r/pics because the most popular Reddit posts are pictures.

Yes, subreddits are forums. I guess there are a lot of rules for posting, but you can easily get a feel for the site without reading any rules by just looking at the content before you post anything.

I think maybe you're focusing too much on trying to understand every detail before you look at the content? When you click on "AskScience" don't read through all the rules, just look at the page. Oh look, a question about how schizophrenia drugs work, that's interesting, I'll click on that. And there's a bunch of comments discussing it. You click on "pics", oh cool, a picture of a protest in Hungary, I'll click on the comments to see what people have to say about it.

Wikipedia has a million rules about editing and maintaining and vandalizing and whatnot, but you don't have to read a single one to enjoy the content. Just look at the front page and click on stuff that is interesting. If you find that the front page has too many funny pictures and not enough science articles for your tastes, then start using subreddits. Unsubscribe from /r/funny and subscribe to /r/chemistry and /r/biology or something. If you're just in the mood for history, go to /r/history and look at that front page. Same as with a news site, if you just want to read world news then you click on the world section. If you want all news then you stay on the front page.