From AMS Glossary
The use of special forms of frequency, phase, or amplitude modulation to permit a radar system to achieve higher range resolution than that normally permitted by a given pulse duration. A suitably modulated transmitter pulse of duration τ (and hence range resolution cτ/2, where c is the velocity of light) may be processed after reception to obtain a higher range resolution cτ/2n, where n is the pulse-compression ratio.
Compression ratios of 10–100 are commonly achieved through the use of linear FM, nonlinear FM, or phase-coded modulation, implemented by analog or digital means. The advantage of pulse compression over simply transmitting shorter pulses is that high range resolution is achieved while maintaining the benefits of high pulse energy.