A ratio of sampling frequency for a digitized video signal. The first number refers to the luminance part of the signal, the second two refer to the chroma (colour).
In this ratio the luminance is sampled 4 times at 3.37 MHz (13.5 MHz total), while the chroma values are sampled twice at the same rate. Chroma is sampled less than luminance because the human eye is more sensitive to brightness than colour, so more importance is given to the luminance part.
The technical meaning of the three values is:
(Red - Luma)
(Blue - Luma)
4:2:2 has been a common standard in television broadcast for some time and the quality is actually very good - more so than the halved chroma signal might suggest.
A ratio similar to 4:2:2, but with the addition of an extra channel for keying.