Single word to describe a person who is not part of a specialized group and is therefore unlikely to understand the vocabulary/concepts of said group. Usually used when asking members of said group to explain a concept to them like they were a _________.
Layman, in sense 2 of the Oxford English.
Alternatively, and only very informally, "explain like I'm five" means "explain in simple terms" (where "five" here means "five years old").
"Put it in layman's terms for me."
in layman's terms ODOL
Phrased so as to be easily understood, without the use of technical or obscure terms.
‘They will explain things in layman's terms and work with you to not only help you reach the top but understand how you got there as well.’
(Layman has already been suggested by Patrick Stevens.) See MW "Layman: a person who does not belong to a particular profession or who is not expert in some field"; "Layman's terms: simple language that anyone can understand The process was explained to us in layman's terms."
This isn't a direct fill-in-the blank solution, but nonetheless it captures the same essence:
Please explain in plain English.
Novice, according to Cambridge Dictionary (2 meanings that might fit):
a person who is beginning to learn a job or an activity and has little or no experience or skill in it
a person who is not experienced in a job or situation
Attribution: (Definition of “novice” from the Cambridge Academic Content Dictionary © Cambridge University Press)