A notice of pendency is an action that affects the title of real property. Generally, the real property has a lien against it. The purpose of the notice of pendency is to alert any potential buyer that they should take caution before buying the piece of property.
If a person buys a piece of real estate that is subject to a prior lien, that individual is subject to the lien and cannot prevent that lien from being enforced. For example, if an individual buys a home for $200,000 from Fred and there is a lien against the home by the bank that holds for $150,000, then there is nothing that will prevent the bank from foreclosing on the home and selling it in order to get its money. The purpose of a notice of pendency is to try and prevent this from happening.