Full Stack refers to two larger areas of effort. The basic divide splits skills and knowledge into what's called the front end; the things people see on a web site and the back end; what people don't see but is required to serve up web pages.
The backend Server and the frontend User Interface.
A backend developer needs to know how to get a server working by installing the right software. There are plenty of decisions to be made including which operating system, which programming languages to support, the database system that'll be used and other software that helps with deployment and version control.
A Full Stack developer is, by nature, a generalist. They are smart to use trusted solutions supported by large communities. This community becomes the reference when something breaks. Not if, when.
The work of a backend developer is never seen by people visiting the site. They will, however notice if the site is slow or if it's broken. This is when their pager alerts go off.
The person working on the front end has a whole set of headaches that also require a community. Originally, a web developer was creating pages a lot like you'd create a printed poster. You just laid out the design and let people move it around the screen when their display wasn't the size and shape matching the designers.
Nowadays, there are multiple device sizes and various browsers that all implement web technologies a little differently. Imagine a baker having to worry about wheat and peanut allergies along with trying to create delicious treats. Lots of workarounds are required.