Most of the time Redemption is used at the beginning of a team fight. Because of its large radius effect, you can easily hit multiple enemies/allies without having to pick the perfect cast area. You cast the Redemption at the beginning of a fight because it takes 2.5 seconds to hit the targeted area. And within 2.5 of a teamfight, a lot of stuff can happen: allies getting crowd controlled and damaged, etc. Your Redemption is here to heal up the team, so that they can either keep fighting, or avoid death.
Another use of Redemption is to save one or two allies that are far away from you. Redemption has a cast range of 5500, which is pretty huge; it allows you to heal allies from very long distances. The thing about this is that, if your low-health allies are running away from enemies, you don't want to "trap" them by placing the Redemption area at the wrong place: they will stay in the area and the enemies would be able to reach them, and kill them. You want to anticipate your allies movement and the 2.5 second-delay of the Redemption.
And yet the ultimate way of using Redemption, is to "snipe" down the really low-health enemies that ran away from your team. But if you're new at the game, I recommend only using the first two methods.