Burning Mook has 2 Attack and 1 Heart. Additionally, it is completely immune to fire-related cards, such as Fire.
If Burning Mook is on the field, at any time the owner may sacrifice Burning Mook to deal 3 damage to another card or their opponent if they're undefended.
Because Burning Mook's effect states "at any time", it is very possible to play Burning Mook and then immediately sacrifice him to deal damage to a card or player. Burning Mook only has 1 Heart, so it is very likely it will be destroyed quickly (and give your opponent 100 points), to effectively use Burning Mook one should use it's ability as early as possible.
Alternatively, if you can convince the mod that an enemy's card attacks with fire, Burning Mook should provide a good counter against it.