A model release for gives writtern permission for the photographer to use the images that are taken during a shoot and dictates the terms the images can be used.
More often that not release forms permit the photographer to do almost anything unless the result would cause defamation.
It's important to note the model signing the release form is required to be over 18.
Models under 18 will need a parent or guardian to sign for them.
For commercial use often release forms are more complex and can dictate right usage in terms of where the image can be used, for how long and who by.
It ultimately serves as a written contract that protects both photographer and model so both parties know how the images can be used.