To describe something to someone in Layman's terms means to describe something in simple ways so that the average person can understand what they are talking about.
A Layman is basically someone ore a person without professional or specialized knowledge in a particular subject that is being described.
If someone is describing something to you in Layman's terms they are trying to explain a complex thing to you so that you can easily understand what they are talking about.
Some things are really hard for some people to understand because some people cannot wrap their brain around what is being said.
So someone will try to dumb down the problem and describe how something works in a more simple way.