It is when you hook two amps together to power your sub(s). For example. If your amp puts out 1000rms @ 1OHM. Strapping two of them would put out 2000rms at 2 ohms. Each amp will see 1 ohm of impedence in that situation. Not all amps can be strapped though. If you amp can be strapped it will say linkable/strapable in the instructions and there should be a diagram of how to do it.