When a creature and at least one of its allies are adjacent to an enemy and on the opposite sides or corners of the enemies space, they flank that enemy and each of them receives +2 to Melee Attack rolls against that enemy. Flanking a creature on both axis' grants a +5 Melee Attack roll.
You can’t flank an enemy that it can’t see and all allied creatures flanking the enemy must be able to attack unimpeded (stunned, prone, blind or incapacitated allies do not count towards flanking).