Concentration 5e

Just as swordsmen rely on their skills with a blade to keep them alive, the lifeline of spellcasters is their ability to harness magic. Certain powerful spells require more than just the ability to manipulate the arcane arts, they require the user to continually maintain control over the spell’s duration. 

Concentration was introduced in 3.5 as a way to regulate powerful spells with multi-turn durations. But what exactly does it mean, and how does it work?