What is Improper rotation axis?

Example 4.6 (Elements of four molecules)

$\ce{H2O}$: $E$, a $C_2$ axis bisecting the H–O–H angle, and two vertical planes, the molecular plane and the plane perpendicular to it. $\ce{NH3}$: $E$, a $C_3$ through N (operations $C_3$ and $C_3^2$), and three $\sigma_v$, each containing one N–H bond. $\ce{BF3}$: a $C_3$, three $C_2$ along the B–F bonds, $\sigma_h$ (the molecular plane), three $\sigma_v$, and an $S_3$. $\ce{CH4}$: four $C_3$ along the C–H bonds, three $C_2$ bisecting H–C–H angles, which are also $S_4$ axes, and six $\sigma_d$, each containing two C–H bonds (figure below).