Q is short for the ‘quality-factor’ of a filter, which defines its bandwidth in relation to its centre frequency and indicates a filter’s resonant properties. The higher the Q the more resonant the filter, and the narrower the range of frequencies that are allowed to pass. A Q of 1 is equivalent to bandwidth of 1.38 octaves, while a Q of 6 is equivalent to a bandwidth of just 0.25 octaves, and a Q of 0.4 is equivalent to a bandwidth of 3 octaves.
The Q control on this FabFilter plug-in determines the width of the 'Q'.