ODD functions are such that their graph have symmetry with relation to the origin, namely, that once the graph hits the origin, its behaviour is the same as the other side but in opposite direction, so in essence the left-side if a mirror image of the right-side but upside-down.
Another way to think of ODD function is that, for every (x , y) pair there is a (-x , -y) across the origin. Check the picture below.