Junior developer is someone who was unable to negotiate a stronger position for any reason, such as experience, recent unemployment, expertise in an outside specialty, company is super tightwad about 'status' and/or pay levels, naive about negotiation, etc. Conversely, sometimes you'll also see someone at a 'senior' level because the title meant more to them than pay or other benefits. Sometimes a title distinction is engineered by HR and the hiring team at a company to function that way.

It is often thought of as a prediction about what level of contribution is expected, but that's basically a farce. In every organization I've seen that makes distinctions between junior and senior developers, the only thing different between them is that senior developers are paid more. They don't do more, manage more, design with more foresight, or anything like that.

The exception is that developers who are brand new to a job function as junior engineers until they get calibrated and learn some parts of the codebase in which they can be effective. But this is just as true for a brand new senior dev with 10 years experience in your company's primary domain application as it is for a kid straight out of a bachelor's program whose only prior work experience has been theoretical REUs or something.

Basically, it's just a status tool.