A flank attack in Civilization V is a battle maneuver in which several melee troops on one side surround and attack an enemy - the attack then receives a 10% bonus to combat strength. In order to activate the Flanking bonus, move at least 2 units onto tiles that are adjacent with the same enemy - every melee attack from any of these units will then benefit from a Flanking bonus. It doesn't matter exactly what tiles your units are occupying around the enemy, as long as they're right next to it. Flanking stacks with the 15% bonus to combat in the Honor policy tree for units on adjacent tiles.