"You know what I mean" is a phrase used by the inarticulate and dimwitted (1) as a comma, (2) to allow the speaker to stall for time in order to come up with other connecting words, (3) to buy time and cover the speaker's lack of knowledge of a subject on which they have decided to speak and the resulting insecurity this engenders. 
It is particularly irritating in athletes, as they are paid way in excess of the average salary of their audience precisely to tell them what it is they mean. The unspoken assumption is that the audience are already well aware of the 'gem of wisdom' or 'fact' imparted by the 'expert' and that there really is no need to provide such insightless insights.
"You know what I mean" has evolved into "You know what I'm sayin".  It is primarily used to ask if the recipient of a statement understands what was being spoken.  It is also used frequently through an educating conversation.  For example: The carbon-14 atoms that cosmic rays produce, coalesce with oxygen to structure carbon dioxide, which plants take in naturally and integrate into plant fibers by photosynthesis. You know what i'm sayin? It can also be used as a mating call.  For example: Hey girl, why don't we go to my place and connect our genetilia, you know what i'm sayin?