Option Pending

At a guess, I'd say that someone has a signed contract giving them the right--if they want to-- to buy the place within a certain length of time, for a specific amount of money.  Sometimes they're also living in the place, meanwhile, paying rent. It would be hard for anyone else to buy it while that was in force.  You'd need the first person to give up that option.  They probably wouldn't have to.  They could either decide to buy, or perhaps just chose to keep the option going for however long it was good for. 

On the other hand, I may be wrong.  Perhaps something else is going on.  You'd have to ask more questions of the agent, seller, or lawyer who said that.