What does restriction U mean on a washinghton state drivers license?

It applies only to Commercial Driver Licenses (CDLs). "U" is the code for "intrastate only" (as it pertains to operation of a Commercial Motor Vehicle). It's given to people who either get a CDL when they are below the age of 21 or who self-certify themselves as "intrastate exempt" or "intrastate non-exempt" drivers.