What does it mean to create a "yardstick" build in an RPG, specifically in Neverwinter Nights?
A yardstick is a definitive standard. Yardstick builds (the term is used more often in CRPGs than in RPGs but it's as valid a concept there) are considered definitive standard builds for a given character class/type. For example, "here's a yardstick build for a two-weapon fighting rogue." Also known as a reference build, prototypical build, benchmark build, or canonical build.
These are differentiated from many CharOp kinds of builds because though they are somewhat optimized for their roles they are generally not completely cheese-weaseled out, and are generally single-class.
Their use is to judge the efficiency of a given character against the standard reference. "I built a dagger wielding fighter! I put all the juice I could into it but he still does 50% less damage per round than the reference build."