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 chroma is sampled once at 3.37 MHz. 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)
As you can see, there is no green in the signal. This system uses a clever technique of calculating the green value from differences in red-luma and blue-luma.
4:1:1 is used by a number of digital formats, including DV, DVCAM, DVCPRO25 and MiniDV.