The icon in the bottom left corner represents a Mercenary’s Background (Special Forces, Airborne, or Naval Infantry).
Each Mercenary’s Background affects how it fights against enemy Mercenaries. Your Mercenary can receive a 25% Skill boost if their Background is more effective against an enemy Mercenary's Background (Naval Infantry has the advantage over Special Forces, Special Forces over Airborne, and Airborne over Naval Infantry).
So if your Mercenary's Background is Naval Infantry, and it is fighting against a Mercenary with a Special Forces Backgroound, it will gain a 25% Skill boost.
If both your activated Mercenaries have matching Backgrounds, their skills will be boosted.