Help: Ratings
All posts on Bronibooru are one of three ratings: Safe, Questionable, and Explicit. Safe is the default if you don't specify one. There should never be a reason to post explicit images on Bronibooru. Please note that this system is not foolproof: from time to time questionable images will be tagged safe, and vice versa. Therefore you should not depend on ratings unless you can tolerate the occasional exception.
Explicit
Any image featuring nudity, sex, or severe bodily harm causing massive blood loss.
Safe
Safe posts are images that you would not feel guilty looking at openly in front of the target audience of My Little Pony (young girls). If that girl's mom wouldn't let her see the image, you shouldn't mark it safe.
Questionable
Basically anything that isn't safe or explicit. This is the great middle area, which should be reserved for (possibly) adult topics without going so far as to be explicit.
Search
You can filter search results by querying for rating:s, rating:q, or rating:e for safe, questionable, and explicit posts, respectively. You can also combine them with other tags and they work as expected.
If you want to remove a rating from your search results, use -rating:s, -rating:q, and -rating:e.