Actual work experience, presumably in whatever you want them to be experienced in. E.g. If a job requires you to use Java then you’d need work experience in Java. Things that are very similar may also qualify. E.g. Someone knows JS and job requires TS.

But maybe I’m misunderstanding the question. If we’re talking more like what makes a developer a senior developer then it’s completely different criteria.

Knowing what to build yourself, and what to leverage from existing partial solutions. Knowing how to mentor other developers and give meaningful PR feedback. Being able to plan and execute a technical project from start to finish on your own if you had to. Being knowledgeable about potential pitfalls before you hit them.