The practice of swapping out different brands of a particular type (say, KT88 or EL34, to name two common types) of tubes in an audio amplifier.
Being the component in an amplifier that actually amplifies the signal, the tube chosen can have quite an effect on the sound. Because of different quality control and manufacturing techniques from one company to another, simply swapping out the tubes in your amplifier can change the sound.
Naturally, all of this is very subjective to the tastes of the particular listener, but objectively some tubes are more linear than others and so should provide more accurate sound reproduction.
Tube rolling, then, can be thought of as auditioning different tubes, much the way you audition different amps or speakers themselves in a store before buying them.
The tubes that came with my stereo sounded really flat, it's time to do some tube rolling to see if I can get a good set in there.