A walk-away price is the lowest version of a deal you will accept before it stops being worth doing — the point below which saying yes hurts more than saying no. It is not the same as list price; it is the line you draw based on your costs, your leverage and the value you are giving. Knowing your walk-away number in advance keeps negotiation calm and concessions deliberate. Without one, emotion writes your counteroffer.
In plain English
Before any negotiation, decide what the floor is. That way a discount request is a yes-or-no against a number you already chose, not a guess made under pressure. The walk-away price protects you from agreeing to a deal that loses you money or goodwill.
Why it matters to you
Deals that look won but lose money are losses. Reps who set a walk-away price in advance anchor higher, concede slower and walk away with respect when the numbers do not work. It also makes you credible: buyers respect a seller who says "that one is below what we can do" and means it.