Why QRP comes from the Q-signal QRP, which means "reduce power" (QRO means increase power). Hams have adopted it as shorthand for operating deliberately at low power, conventionally 5 watts output or less on CW and digital modes, and often 10 watts PEP or less on SSB. QRP operators rely on good antennas, efficient modes and propagation to make contacts with a fraction of the usual power.
Watch out The invented "Quick Response Protocol" is a made-up expansion; QRP is a traditional Q-signal, not an acronym for a data protocol.
QRP = Reduce Power, QRO = Ramp it Over. 5 watts is the classic QRP CW limit.