Ren, okeaki is japanese for "doodle" or "sketch". Okeakis are meant to be drawn on simple paint programs using only the very bare basics and no fancy effects or options, making it therefore a simple sketch or doodle, an okeaki.
As simplistic as they are, sometimes they're actually a challenge to draw, especially for people who depend so heavily on things like photoshop. I've seen some really good okeakis which one would think were not okeakis unless someone would have told you so.
Okeakis are usually done in MS Paint and other such outdated programs.