After completing a character's loyalty mission, you can unlock a new skill for use by the main character. To pick or change which skill you are using, simply complete the "Advanced Training" research upgrade on your ship. On your first character, you will need to train to use these skills, but on every character made or imported after unlocking these skills you can pick them immediately upon creation.

Unlockable	How to unlock
Biotic Barrier	Complete Jacob's loyalty mission.
Geth Shield Boost	Complete Legion's loyalty mission.
Inferno Grenade	Complete Zaeed's loyalty mission.
Reave	Complete Samara's loyalty mission.
Shield Drain	Complete Tali's loyalty Mission.
Slam	Complete Miranda's loyalty mission.
Warp Ammo	Complete Jack's loyalty mission.