Hi,
Basically, the beneficiary of the usufruct is allowed to use and enjoy the property as they wish for the rest of their life. The parties who will eventually inherit the property are called the bare bones owners of the property. As the property can be used as the usufruct beneficiary wishhes, including renting it, the bare bones owners can only visit if they are given permission. They are also financially responsible for the IBI, community fees and extraordinary repairs and maintenance.
Also, if the bare bones owners and the beneficiary were to agree to sell the property, the beneficiary would be entitled to a substantial share as Spanish law assigns a financial value to the usufruct.
Usufruct is very common in Spain because of the inheritance laws regarding spouse and children and the proportion of the estate which can be bequeathed to them. It can also be a very tax efficient mechanism if you know how to use it apparently!