What does 18mm/20mm watch band mean?

It means a watch-strap 18 to 20 millimetres wide.   Approximately 3/4 of an inch. Three quarters of an inch. Three fourths of an inch.

Clearly 18 and 20mm are not quite the same size, but there is only one twelfth of an inch ( 1/12 inch ) between them, so probably not worth thinking about. 

You're probably obsessed with having big black dick in your ***. It just shows. 

Hey ***** come to toledo and shut me up Eric.  Come to toledo I'll face you in a no holds  barred fight. Your family, friends and you are in danger.  I have people looking for you 24 / 7. 

Step up *****. What's your number

Help me Eric I'm mental and retarded I like to rub my feces on a turkey.

Hope your mother gets raped and stabbed in front of you 

I think the 18 mm and 20 mm are two possible widths *across* the watchband as it is lying flat on a table (with or without the watch on it at the time). Some watches are designed so that the 'tension bars' on either side will fit a 18 or 20 mm band slipped through them. (I am wearing such a man's watch right now - the plastic-fabricky strap goes over and then down under, across the *back* of the watch, and comes up and out the other side, crossing over the second 'tension bar' on its way around your wrist.) It's a band-maker's recommendation that 18mm or 20mm width probably fit the best, that a 22mm band probably won't fit in the space next to the 'tension bar' at all, and that smaller mm widths may not look as good; it's meant to help you buy the best band for the watch. (I have no idea what these 'tension bar' thingies are really called; I just know that they're the devil to try and take off by yourself without some good tools to do so.)