flow as in menstrual flow, the literal volume of blood and mucosal lining from the uterus during a woman’s period

flow also as in lyrical flow, the use of rhythm and articulation in a rapper’s delivery

bar as in a measure of time in music, typically a four beat bar

spitting referring to the rapid and rhythmic delivery of words to a beat

If your [lyrical] flow is so heavy, why aren’t you [rapping]?