A headshot is a professional photograph. Looks don’t typically matter in most job fields, but if a casting director has a certain type of person in mind for a role, they will gravitate more towards a candidate that fits that physical description.
This means that otherwise arbitrary information such as your height, weight, eye color and hair color will be more important to whether or not you land a specific role. As such, you can expect the format of an actor’s resume to look somewhat different compared to a resume for another job.
Not all acting job applications will call for a headshot, though you should have a few prepared beforehand just in case.