It's a loaded term, similar to DevOps, that can mean different things depending on the person/company using it. In general, it is a dev who can code/dev throughout the full stack of layers in the process. This is opposed to a dev who focuses only on one aspect of software dev. They do both front and back end work.
Not exactly something one just walks into.
It's also something companies use to attempt saving money and drastically underpaying developers.
"Here, take on these 5-10 jobs for the low end salary of one job!"
"Um... I have 15 years experience"
"Yeah, but only 1 year working with Java 9, and that's a big part of the job"
"Java 9 was only released in September 2017 you moronic piece of HR trash"
"So, we can offer $1k more than the original offer"
"**** off"