Will one (or more) of you webmaster geniuses please explain what "URIs must be properly escaped" means?..or at least refer me to an appropriate resource?

This probably is something very basic but I am unable to find the answer in my HTML book right now. This is one of those late night quandaries keeping me up even later trying to solve the mystery while I am cross-eyed from looking at code. :notsure:

Serious and the not-so-serious replies will be appreciated.