Sirens are a mystery in the Borderlands universe, as there can only be six of them at any given time and they all have incredible powers. Maya was identified as a Siren as an infant, and then handed over to an order of monks on her homeworld. She trained these powers of hers until she found out that the monks were using her to get money and obedience from the people of her planet. She killed her handler, a Brother of the monk order, then moved to Pandora for both adventure and to learn more about Sirens.
Maya's action skill is called Phaselock, which allows her to suspend foes in another dimension. This is represented on the screen as enemies being trapped in a bubble, rendering them immobile and unable to do anything for the five seconds it lasts. It has a fairly quick cooldown time of 13 seconds off the bat, it can be reduced by three seconds with Quicken (Thoughtlock increases it by three seconds, though). Note that some enemes, like bosses, cannot be Phaselocked, so they take a set amount of damage instead.
While it is invaluable during intense firefights, Phaselock can get especially dangerous once you dabble into some of the skills that enhance it. Helios makes it so surrounding enemies, as well as the one Phaselocked, will receive fire damage. Converge will pull surrounding enemies towards the one Phaselocked and deal some damage. Ruin, one of the Gamechanger skills (bottom of skill tree), applies corrosive, slag and shock effects on the enemy Phaselocked, as well as surrounding ones. Combine all three and enemies will have all four elements on them.