A model release is a written agreement between the model and the photographer where the model has granted the photographer permission to use the photographs commercially and/or for self-promotion. Model releases generally permit the use of the image(s) for all purposes, with exceptions for controversial, sensitive or defamatory uses.
You should discuss the photographer’s model release prior to your shoot — and if possible, view a copy so you will know exactly what you will be signing. If you don’t agree with any parts of the contract, discuss modifying the contract.