I guess they mean you need to identify specific people, at a specific place and time, who are desperate for a solution to some problem.

They are desperate usually because it is a problem that affects their bottom line, prevents them completing a project at work, wastes an enormous amount of their time when doing something important to them, etc.

It must be a solution that, even when distilled to its very core, provides clear value to specific people you can identify. Just lighting on a vague market need or undeserved niche is not enough.

Not to say even purely passion projects can't succeed, but those are more hit-or-miss.