The [CII] 158um line, aka ionized carbon fine structure line or ionized carbon forbidden line, is a spectral line produced by a small (fine structure), rare (forbidden) transition in singly ionized carbon. It is within the far infrared.
As far as I can make out, carbon is ionized at a similar energy-absorption amount as is hydrogen, so where hot, short-live, stars produce ultraviolet thermal emission that ionizes hydrogen (forming HII regions), they also ionize the small amount of resident carbon. The [CII] emission line serves as a tracer for the star formation indicated the presence of such stars.
The wavelength is not in an atmospheric window, but from cosmological distances, can be redshifted into windows and viewed from ideal ground locations.