Pyro mine is a ability unlocked upon reaching level 20.
It is not yet confirmed weter this ability (presumbly for the engineer) either buffs the damage, amount of mines you can plant, the damage they take , if it adds any other effects (fire for instance) or supplies used planting them. There could be even more possibilities.
Strangly, it dosn't appear as an ability on the ability menu, nor do you have to buy/equip it.