The long primary season several candidates gain delegates, but not enough to secure the nomination, and they drop out. Rick Santorum is the latest. OK so what happens to his 250 to 280 delegates? Well, that depends. States pick their delegates differently and have varying rules about them.
The clearest article I’ve read on this is from the Washington Post. It starts with Rick having 281 delegates. Of these, 84 are from states with nonbinding delegates: delegates from Iowa, Colorado, Minnesota, Washington, and North Dakota have always been free to vote for whomever they want. You can imagine they have been contacted by other campaigns by now.
The remaining 197 delegates are bound to vote for Santorum at the convention unless Santorum releases them, and of this writing he has not. He did not end his campaign but only suspended it; had he ended his campaign that would have released his delegates.
His decision to suspend his campaign essentially gives the nomination to Mitt Romney. To the extent that Rick did not have enough delegates to block Romney’s election, Rick’s bound delegates will likely go through the motions of voting for him in the convention.