A permission set in Salesforce is a named bundle of permissions you assign to users on top of their profile, so one person gets extra access without changing everyone else who shares that profile.
Think of the profile as the key every employee gets on day one, and permission sets as the extra keys handed to the people who need one more door. A custom permission is one of those doors that you build yourself, and your formulas, Apex and Lightning web components can check whether the user holds it.
We built both in a Developer Edition org for Ridgeline Supply: a Ridgeline Invoice Manager permission set and an Approve Freight Discounts custom permission. By the end you will be able to create, deploy and assign a per