Their exercising their right to not answer a question in a court of law because their answer might convict the defendant. its basically same as the "right to remain silent" in one's Miranda rights, except it is used while they are under oath, while miranda rights are used during the process of arresting. People who dont use it in a court of law can use it the same way, in that if they answered a question truthfully, it would make them guilty, and example of "pleading the fifth"
"Husband, did you have sex with another woman?"
"i plead the fifth"
and there you go.