A title is a legal proof of ownership. When the buyer signs a sales contract one of the standard clauses is that the seller will provide the buyer with a title that is free of liens and that the seller has the legal right to convey. This means that the seller must own the real estate and that no other person or entity can have a legal interest in the property; the proof of this is called the title.
These legal interests can be other people who legally own the property or, more commonly, liens on the property. If the seller has not paid certain bills, such as those for home improvements or for taxes, the company the money is owed or even the IRS will file a legal lien on the home. The seller cannot prove a title to the property until that bill is paid off and the lien is removed by a court judgment.