It most likely means that the seller has an offer on the house but that the buyer needs to sell his house in order to purchase the sellers house. This is called a offer with a contingency. Usually, in this type of offer the selller will accept the offer, agree on a closing date but if the buyers house hasn't sold before than the seller can still accept other offers. It could also mean that the sellers have an offer but maybe the buyers credit seems shakey. Hope I helped you and didn't confuse you.