A Stock Character is a character who is instantly recognizable to us from other stories; the gruff grandpa, the snooty cheerleader, the bratty younger sibling. You can sum up their role in the story in a sentence or less and people will know exactly what you're talking about. Such characters are frequently one-dimensional in nature. Sometimes that's okay. Sometimes that opens the door to Playing With Tropes as well. Sometimes two or more character types get combined as well.