That's about a thousand save game files. It's accessible via the XMB under Games, but it's only for PS Plus users. It's just one more perk on the pile.
Games that are saved via Online Storage can be accessed by any PlayStation 3 through the respective PlayStation Network account, so you can transfer saved games to a friend’s PS3 console to keep on playing - great news for RPGs and the like.
PlayStation Plus subscriptions cost £39.99/€49.99 annually or £11.99/€14.99 for 90 days in Europe. You also get access to other content like beta invites and such.