Thigh cuffs are bondage cuffs designed to fit around the upper thigh.
In general, as a bondage item they are not very effective. The problem is that thigh tapers from the top of the leg to the knee. This means that unless the thigh cuff is attached to a fixed point above the thigh, it will tend to slide down the leg and come off. A common variant available in a number of stores is the wrist to thigh cuff, where a wrist cuff is directly attached to the thigh cuff. In theory, this allows someone to be bound with their wrists held against their thighs, allowing them to stand or walk. In practice while they may look attractive, they are almost totally ineffective as a bondage device as they come off far too easily.
Thigh cuffs do have a use when someone is tied in a vertical spreadeagle position or against a St Andrews cross. In this case, they can be used to help support the weight of the body and lessen the tendency of the bound person to hang from their wrists.
The disadvantages can be counterbalanced when wrist-to-thigh-restraints are combined with arm-to-waist-restraints, for example.