The page for space group P2₁/c from the “International Tables” is reproduced below:

Key: By convention, the a-axis is drawn in the vertical direction; the b-axis is horizontal. The characteristic symmetry elements are listed in the order of the cell axes a, b, c. In this case, there are no symmetry elements parallel to a or c. Thus, there are “1” 's in the first and third positions. The “21/c” in the second position stands for a 2-fold screw axis parallel to b and a c glide plane perpendicular to b. Translational symmetry elements cause entire classes of reflections to be absent. In P21/c, the c glide plane perpendicular to b causes h0l reflections to be absent when l is odd. The International Tables actually lists the observed reflections. Thus, for h0l reflections, only those with l even are allowed. The 2-fold axis causes 0k0 reflections with k odd to be absent. For an example of a molecule in space group P21/c, go here.