My textbook defines $\mathrm{t_{2g}}$ orbitals as
$\mathrm{t_{2g}}$ stands for a set of three orbitals which are asymmetric with respect to $C_2$ axes, perpendicular to the highest $C_n$ axes, but which is symmetric in sign through the centre of inversion.
I know that “$\mathrm{t}$” is a randomly chosen letter as suggested by M. Farooq, “$\mathrm{g}$” comes from gerade (MOT), that's what the last line says. Though “$2$” in subscript must have come from $C_2$, but I didn't get the meaning of
…which are asymmetric with respect to $C_2$ axes, perpendicular to the highest $C_n$ axes.
Please explain what does this line conveys.