Any mech struck with Burn, will take additional damage every round for the next 4 rounds.
Sample: An inferno hits for 100 damage with a weapon that has 8% burn. The opponent mech will take the 100 damage and an additional damage of 8 points per round every one of the next 4 rounds.
Ignite: When a mech affected by the burn effect (taking already burn damage) gets stroke again by a burning weapon/mech, the burning mech will "ignite", receiving furious extra damage for the next 4 rounds.
Traditionally, the weapons with burn skill are fire type, but some rare lasers or cannon could provide that effect.