To put it simply, 4chan is a like the dump of the internet.
Everything is on 4chan. And it's never anything of importance. It's mainly a huge image board/forum. From time to time you will find someone on there who is actually helpful and/or nice, but most people are intentionally mean because you can post anonymously.
If you have a weak stomach (ie nudity, racism, blood are very prominent), I suggest you stay away.
I usually stick to /cgl/ which is for cosplay and egl, because a lot of amazing pictures are posted there. But almost everything posted will be called fatty/anorexic/ugly/bad costumer etc no matter what or who it is.
Definitely good for references and stuff though, and the home of a thousand and one memes.