Predistortion is a technique used to improve the linearity of radio transmitter amplifiers.
Radio transmitter amplifiers in most telecommunications systems are required to be "linear", in that they must accurately reproduce the signal present at their input.
An amplifier that compresses its input or has a non-linear input/output relationship causes the output signal to splatter onto adjacent radio frequencies.
Likewise there are two equivalent ways of conceptualizing how predistortion amplifiers work: correcting gain and phase distortions, or cancelling inter-modulation products.
Usually one of the two conceptualizations is preferred when designing predistortion circuitry; however the end result is generally the same.