That's Soraka's passive. It increases ally's magic resistance by 16 in a 500 radius.
There two main problems with it:

1. The bonus amount is flat. It never increases and it's barely noticeable in the late-game. Supports are very reliant in their skills to be effective and it's almost a skill slot gone to waste, considering late-game numbers.
2. The range is terribly short. Heimerdinger's Techmaturgical Repair Bots has a range of 800. Most aura items have a range of 1000~1200. 500 is just a bit more than Sunfire Cape's.

Either it should scale into late-game, making it so allies near Soraka fell like they're in an anti-magic sanctuary or the range should be increased, so Soraka doesn't need to be leading the charge for the buff to affect the ones it matters most.

MASTER EDIT: Just gotta thank whoever fixed Consecration's radius at the wiki. I tested it and I can confirm it has indeed a radius of 1000 (and diameter of 2000). http://leagueoflegends.wikia.com/wik...&oldid=1658459

isn't 16 MR pretty strong early game?

if they gave it scaling it would have to be nerfed early coz its quite good early

The range of Soraka's passive is 1000, not 500. As to specifics about the passive, it is potent and actually scales indirectly -- MR is % damage reduction and teams tend to group up more and more as the game progresses. 16 MR may not seem like a lot, but 16 MRx5 is probably stronger than a number of champion passive, especially when you consider how much random magic damage bruisers, tanks, and other supports (champs that build zero/low spell pen) tend to put out. There's a reason why Runic Bulwark is bar none the most powerful item in our game.

With that out of the way, the passive is lacking in gameplay. Even when playing as Soraka, I seldom appreciate it and definitely cannot feel the difference it makes in a team fight. Contrast with Nami passive -- likely a lower power passive overall -- whose speed boost you and your allies can't help but notice. Soraka players never get to feel cool or clutch, to display skill, for their passive in the way Sona does with her Powerchords or Blitzcrank does with Mana Barrier-enabled baits or dives. I'd rather we give Soraka a new, more engaging passive, rather than buffing the current version.

Soraka goes botlane against an AD carry, so the passive isn't all that great. Her armor buff is great though.

What you could do to help feel the passive is to improve it visually. I'm talking redesigning the icon to picture some sort of light blue rings given to allies in range. That would give you the satisfaction to know it's there helping by seeing something. Just my 2 cents