What does it mean to be specilized web developer?

Does it mean you focus on one language only or you can create/fix only one type of app? are there developers out there who can only do one thing and are not very knowledgeable about anything else but still get paid because that thing is popular? How easy is to adapt to another type of app than the one you are specilized at?

I ask because clients hire developers, they don't specify what specilization they should have except the language maybe.

When I start creating stuff, should I focus on one popular thing? am I supposed to be good at everything that involves my programming language If I want to find work?

This one might help you: https://github.com/kamranahmedse/developer-roadmap

I would suggest you start playing with wordpress,
it's very simple to work even for someone with no programming experience, you can make a usable website from day one.

Web development (means Frontend development) based on such language as JavaScript (and ES6 at the last time) and following popular libraries and frameworks: JQuery, Angular, React, Redux and so on.
The more your stack of technologies, the more you will be in demand.