Where a photograph or illustration contains a recognisable image of a real person then that person signs a 'model release' to authorise the sale of the image for commercial use. Where we indicate 'Model release on file' for an image this means that we have a model release for that image on file.
Other subjects may also require a release, called a 'property release', to authorise the selling of their image. Subjects which require a property release can include some buildings, brands, designs and works of art. Where the creator has provided us with a property release for the image this is indicated by 'Property release on file'.
Please note that in most cases editorial images will not require a model or property release because you cannot use them for commercial or promotional purposes.