A fee tail is an interest in real property that must be passed on to a person's heirs. So, if someone inherits, or is given a piece of property with a fee tail, it means that they own the property, but they must pass it on to their children after death.
The concept of a fee tail has been abolished in most places in the Untied States.
Fee tails have largely been replaced with fee simples. A fee simple is an interest in real property that is complete and undisputed. So nowadays, in most jurisdictions, if a person leaves a property to someone and their heirs, it usually means that the person gains ownership of the property "in fee simple." Once they own the property, they can decide if they would like to pass it along to their heirs or not.