It means that enough has been said and the person has gotten to their point.
"Let's go out to eat!"
"Can't, I'm broke."
"It's on me."
"SAY LESS!"
Say less means The user under what you are saying there is no need to further explain.
1. When you understand someone fully and clearly and you don't need any of their further explanation.
Tyler: Hey, dude. Did you know that I saw Carrie leaving my house party with Drew?
Ronald: Oh, really? Say less; I'm going to have a real, long talk with her.
Derived from "say no more". Used when something is fully understood, accepted, and/or agreed to.
Victor: Hey man, I need at least 20 points and 10 rebounds from you in the big game tonight.
Nick: Say less fam, I got you.
Brittany: Girl I just heard Vanessa over there telling people that you were musty in gym class earlier.
Susie: Say less, let me go see what her problem is.
Say Less means "l understand you don't have to say anything else to help me understand"
Burhan: yo Drake just released his new album
Jason: Say Less fam
It means that someone says something good that you are excited about.
Jack: Yo dude I bought you some ice cream.
Sam: Say less!
or
Parent: We're going to Disneyland for Christmas!
You: Say less!