As most other addictive games HexaMon is easy to learn but hard to master.
What you do is rather simple:
Group four identical monsters so they are next to each other. Do this by selecting a monster and movíng it to another place (with a free path inbetween).
If you succeed all monsters will disappear and one higher level monster will appear. The level of the monster is recognizable by looking at the number of eyes and/or the color of the body.
But be aware there are two different monster families not being compatible to each other. They differ in many aspects including body shape, haircut, teeth, legs etc.
In case you join a group of monsters of the highest level all monsters will disappear.