Reddit's pretty simple. Make a headline that doubles as a link to either text or something else (a site, image). It gets upvoted or downvoted based on interest, relevance or (it isn't supposed to be, but it does get rated on) whether people agree/disagree with the idea therein. Some people put a lot of stock in the total number of upvotes they've received to date, called 'karma'. There's also 'gold', which is bought by redditors and awarded to particularly deserving posts, helping to fund the site while giving the person who got gold some convenient bonuses for effective browsing (see more on one page, see what you've already viewed, etc).
Based on the number of upvotes or downvotes vs. the time it was posted (early upvotes matter more than late ones), stuff rises and falls in the lists. This includes comments & comment strings. Something with no upvotes will disappear into obscurity, while something with a lot of upvotes can make it to the front page, which means 100,000+ people might check it out. The default frontpage (ie. the page you see when you visit reddit.com) is basically limited to the 10 or so subreddits with the most subscribers. Funny, Gonewild, adviceanimals, IAMA (I am a ____, ask me anything), pics, science, atheism, and so on.
Reddit is divided into subreddits, each one focusing on a different subject (or different perspectives/approaches to the same subject - there's something like ten . There's a subreddit for just about everything except the most reprehensible stuff (the jailbait subreddit got closed down around the time I joined, IIRC). You can subscribe and unsubscribe to include or filter out specific headlines. If you're a big user of Reddit, you might want to use the enhancement suite, which gives you more freedom, data, and (something I like) the ability to filter out certain stuff. Good for avoiding spoilers or screening out the more annoying junk.
Some subreddits are maligned by the community. Men's Rights ("We think guys get screwed over by society sometimes") and Childfree ("We don't want to have kids"), for example, get weird looks and strangers visiting only to downvote them that are pretty undeserved, IMHO, while others like TheRedPill ("Being a jerk alpha male will get you girls"), Spacedicks ("This guy just mutilated himself below the belt, gallery inside") and SRS (Shit Reddit Says, extreme feminists saying, "Let's mount downvote brigades against perceived misogyny & jerkishness!") are despised and/or considered creepy for perfectly just reasons.
Reddit is something of a timesink, and you can easily kill hours just browsing it, but it's also a great way to stay on top of things in your fields of interest. It's not unusual to hear about stuff on reddit before you hear about them by more conventional channels, and if you're interested in anything creative or a particular hobby, Reddit provides a way to keep your thumb on the pulse of the community and get a sense of ongoing trends and interest.
Subreddits are preceded by the /r/ tag. Usernames are preceded by the /u/ tag.
/r/writing is a good place to go for advice on writing or something in that light, but tends to have more inexperienced writers than experienced ones, and a lot of very vague, nebulous questions like, "How do I get over my writer's block?"
/r/shutupandwrite is more disciplined, and doesn't accept that sort of wishy washy stuff, but it's disciplined in a way that makes it hard to get into. Stuff is strictly categorized and many topics are strictly confined to threads that appear on a schedule, like the weekly 'what are you reading?' threads.
/r/worldbuilding is just that. You sort of have to sift through a lot of maps and stuff drawn in MSpaint, but there are a few gems of questions that can get your creative juices flowing or get you thinking about your setting in question. One I liked from the other day was "What are the countercultures in your setting?"
Self promotion will generally get downvoted, but I often browse /r/writing and reply to comments, sharing my own experience. Frequently, people ask for a link to my writing, and I get 20-50 new readers when that happens. Not enough to be worth the time, arguably, but not bad for something I might be doing otherwise.