Layman is a contraction of the words lay and man. In order to use ‘layman’s terms’, you need to be at the same level as the person you are doing it for so that they can understand what you are talking about.
A layperson uses their observations, skills, and experiences in profession or hobby outside an area of formal education (like medicine) to give advice or make judgments without being duly qualified in this area. The term is mostly used when discussing matters pertaining to health care because health care needs to be understood by every citizen regardless of their formal education, which makes it important for doctors providing care not to talk over people’s heads but rather explain topics using plain language and have patience with those who don’t understand.
So, when we use ‘layman’s terms’, we are trying to explain something in a way that is easier to understand and not talk over people’s heads. It means you need to dumb down your explanation so the person you’re talking with can comprehend it.