[grayrest]

There are lots of languages in the world. The majority of them are written by a single person for themselves or (if there's funding involved) a small group of people who deeply understand the compiler. These are either research, hobbyist, or company languages depending on the context of the author. They exist to explore a particular idea or solve a specific problem and don't aspire to be widely used.

If you hear the term used disparagingly it's because they tend to have issues you generally wouldn't want to put up with when you're on the clock: compiler bugs, lack of error messages, large missing pieces in the standard lib, spectacularly bad performance, etc because those weren't the problems the language was meant to explore.