These are simple strap type tennis elbow braces of various kinds which are wrapped around the forearm just below the elbow. Some have additional pressure pads which are meant to sit on the muscle just below the point of pain on the elbow.
They work by compressing the upper forearm and absorbing the forces which are transmitted through the soft tissues to the point of pain on the outside of the elbow. They also change the angle at which the tendon works at the elbow which changes the forces which are applied to the tendon attachment allowing the injured area time to recover.
The advantages of the tennis elbow strap are they are often inexpensive, easy to fit and the level of compression can be adjusted to suit. Disadvantages are they can be less precise than an epicondylitis clasp and do not retain heat like a tennis elbow sleeve. Recommended for most people with tennis elbow symptoms.