A full-stack developer means a developer who can work on all the components related to a web application such as front-end, back-end, etc. A MEAN stack developer is also a full stack developer but a mean stack developer works on technologies like MongoDB for database, Express for the back-end framework, Angular for the front end, and Node.js as the back-end language. So you can say that all MEAN Stack Developers are Full Stack Developers but not all Full Stack Developers are MEAN Stack Developers, because Full Stack developers may also work on other technologies such as Django, Ruby on Rails, etc. Learn the difference between full-stack and mean stack on Mean Stack vs Full-Stack.
You can also read more about this stack this blog on MEAN stack.
You can also get started with MEAN stack by learning individual technologies, like:
If you wish to learn MongoDB you can do this end to end certification course on MongoDB.
Also you can learn Angular by doing this Angular course.