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.
Instead of saying:
"You need a new piston, valve guides, stator, and counter-shaft balancer"
In layman's terms you'd say:
"There are some internal parts that need replaced"
Saying something technical/confusing in an easy to understand manner (like when someone says a confusing phrase, whilst the other is like "SPEAK ENGLISH!")
"Momentum, a function of mass and velocity, is conserved between portals. In layman's terms, 'Speedy thing goes in, speedy thing comes out'"
The use of basic speach, or words/terms that a person will understand.
Teacher: The volume of a container with a gas, when reduced by one half, will double the force exerted by the gas by a factor of two
Student: Uh, I don't understand. Can you give that in layman's terms?
Teacher: When you squeeze a balloon to half it's size, it is harder to squeeze right? This is because the pressure is doubled. Get it?
Student: Uh, Yah!
Something someone slightly slow, like Parker, would understand.
Laymans Terms: Parker has a lot of shit under her arm.