A "layman" or "layperson" is a person who is a non-expert in a given field of knowledge. The term originally meant a member of the laity, i.e. a non-clergymen, but over the centuries shifted in definition.
The concept of describing something in layman’s terms has come into wide use in the English speaking world. To put something in layman’s terms is to describe a complex or technical issue using words and terms that the average individual (someone without professional training in the subject area) can understand, so that they may comprehend the issue to some degree. The term layperson arose to be gender-neutral.
Layman, ‘laity': In short: "laity" means "common people". The English word "laity" comes from the Greek laikos which meant "of the people", "common" (common, in the meaning "unholy", "unclean" and similar). The related verb laikoô meant "to make common", "to desecrate".