Flanking

When a creature and at least one ally are within 5ft of the same enemy on opposite sides, that enemy is flanked. Each of the creatures flanking gains +2 on melee attack rolls against it.

For this to work, you need to be precisely opposite with the enemy between you; the aim is to form a straight line through all three creatures.

Creatures with abilities like blindsight or high passive perception may be immune to flanking.