  The [CII] 157.74 $\mu$m fine structure transition is one of the brightest and
most well-studied emission lines in the far-infrared (FIR), produced in the
interstellar medium (ISM) of galaxies. We study its properties in sub-pc
resolution hydrodynamical simulations for an ISM patch with gas surface density
of $\Sigma_{\rm{g}}=10\;M_{\odot}\;\rm{pc}^{-2}$, coupled with time-dependent
chemistry, far-ultraviolet (FUV) dust and gas shielding, star formation,
photoionization and supernova (SN) feedback, and full line-radiative transfer.
We find a [CII]-to-H$_2$ conversion factor that scales weakly with metallicity
$X_{\rm{[CII]}}=6.31\times 10^{19} \;Z^{\prime\;0.17}\;
\rm{cm}^{-2}\;(\rm{K}\;\rm{km}\;\rm{s}^{-1})^{-1}$, where $Z^{\prime}$ is the
normalized metallicity relative to solar. {The majority of [CII] originates
from atomic gas with hydrogen number density $n\sim 10~{\rm cm^{-3}}$.} The
[CII] line intensity positively correlates with the star formation rate (SFR),
with a normalization factor that scales linearly with metallicity. We find that
this is broadly consistent with $z\sim0$ observations. As such, [CII] is a good
SFR tracer even in metal-poor environments where molecular lines might be
undetectable. Resolving the clumpy structure of the dense
($n=10-10^3\;\rm{cm}^{-3}$) interstellar medium (ISM) is important as it
dominates [CII] 157.74 $\mu$m emission. We compare our full radiative transfer
computation with the optically-thin limit and find that the [CII] line becomes
marginally optically thick only at super-solar metallicity for our assumed gas
surface density.
