refers to when a female is menstruating. crimson means red, as in red as blood. the wave alludes to the emotional waves that many females experience when on their period. hence, riding the waves of emotion while menstruating.
can be used in a negative way. "Dude, she's so moody. She must be surfing the crimson wave!" can also be used in a positive way. "Ebony, do you have a pad? I'm surfing the wave."