It means doing things that you can do when you have 0 to a few users but that would not work beyond that. For example, Doordash validated their idea with posters and manually taking orders and delivering them themselves, no app, no drivers.

It is counterintuitive at first but when you think about it actually makes a lot of sense. When you're getting started you should focus on doing things to get started. Worry about scaling when you're scaling.