*goes to Wikipedia because she's too lazy to think of an explanation* "Oekaki is Japanese for "scribble" or "doodle". On the Internet, an Oekaki is a message board system that revolves around computer art created by a local art program. Oekaki artists often post their art to other websites, such as personal pages or free gallery websites, in addition to the original board."
In other words, imagine MS Paint, except you have somewhat more options such as layers and dodge/burn tools. And then imagine that being on an online message board where you can post your art and other people can see your works and comment on them. That's oekaki.