Subway Footlong

This joke is funny because it's a pickup line that uses a metaphor. It's comparing the size of the listener's "you know what" to a Subway sandwich. The joke is implying that the listener is so good in bed that they can make a 6 inch into a footlong. Now I know that you, dumb dumb, might not know what a 'footlong' or a '6 inch' is. But trust me, if you did, this would be sooooo funny.