QRP means low power. For CW it is commonly taken as 5 watts or less; many people go lower still, to 1 watt or a fraction of it. The Q-code QRP means "reduce power", and the name stuck for the hobby of making contacts on a few watts. In the US the rules also require you to use no more power than needed to carry out the communication.
Why CW suits low power
Cutting power from 100 W to 5 W reduces your signal at the far end by 13 dB. That sounds like a lot, but the other station's receiver only has to separate signal from noise, and a narrow CW filter admits far less noise than the 2.4 kHz filter an SSB signal needs.
Cutting power to 5 W costs about 13 dB at the far end: roughly two S-units, and CW or digital can win it back.Idealised, noise-limited comparison. Real gains depend on the filter, the noise and how SSB power is measured. Illustrative.
The net loss is only about 1 dB in this idealised case, small next to what propagation and antennas change. It assumes noise is the limit. A pile-up of callers, or interference inside your filter, is a different problem, and there real power still helps.
In practice
The antenna matters more than the radio. A good antenna and low feed-line loss help more than anything inside a small rig; see Feed line loss and velocity factor.
Listen more, call less. Answer CQs rather than calling, and choose times when the band is open.
Be patient with pile-ups. A weak signal in the middle of a contest or DX pile-up may not be heard. Call, wait, call again, and be ready to try another time.
Check how you sound. The Reverse Beacon Network will show which skimmers hear you and at what SNR.
QRP is popular for portable operation, because the radio, battery and wire antenna fit in a backpack; see Portable and field antennas.