A substitute (sub)/replacement teacher for a teacher/professor that is absent or otherwise occupied that day/period of class. Generally Americans use the term "sub" or "substitute" instead of "supply". Sometimes the term "supply teacher" can confuse people and make them think its a teacher who gives out school supplies at school, instead of what they are which is an 'extra' teacher available when the original teacher is out.
"Boy, that supply teacher was fit. I hope the next time our teacher is out, he fills in for her."