SOMETHING every student pilot should know is this: When ATC tells you to hold short of a runway, that instruction needs a readback which includes your callsign and the hold point. If you don’t provide one, ATC may repeat the instruction until you do, or specifically request a readback. You should be alert to provide the readback without having to be asked.

As it states in TC AIM RAC 4.25:

To emphasize the protection of active runways and to enhance the prevention of runway incursions, ATC is required to obtain a readback of runway “hold” instructions. As a good operating practice, taxi authorizations that contain the instructions “hold” or “hold short” should be acknowledged by the pilot by providing a readback or repeating the hold point.

But when ATC tells you to contact tower “holding short” of a runway or taxiway, that doesn’t need a readback. Also from TC AIM RAC 4.25:

Pilots may be instructed to monitor the tower frequency while taxiing or until a specific point, or they may be advised to “contact tower holding short.” The term “holding short,” when used during the communications transfer, is considered as a location and does not require a readback.

The following two acceptable exchanges illustrate the difference:

ATC: FLYO taxi charlie delta contact the tower holding short runway 26.
FLYO: Roger, FLYO

However:

ATC: FLYO taxi charlie delta, hold short runway 26.
FLYO: Hold short runway 26 FLYO

Simple, no?