I assume you already know what is MEAN stands for, but a little intro is essential.
MEAN is a technology stack which includes [ MongoDB as Database | ExpressJS as NodeJS framework to write restful APIs, AngularJS as FrontEnd(UI), NodeJS as Backend] (NOT to be confused - NodeJS is backend side javascript and ExpressJS is Framework on NodeJS to write Restful APIs Simply), it helps you to create Web Appliaction quickly.
Learning MEAN Stack becomes very simpler if you have background in JavaScript, As NodeJs is server side JavaScript you can understand and get used to it easily. you have to put a little effort to learn MongoDB if you do not know anything about this DB. Little background in Any NoSql DB can make it easier to learn.
If you wants to get start with latest version of AngularJS 4 or so, i suggest to have a look in this link. which is a step by step implementation of MEAN stack using Angular 4.
Hope this will help you.
Happy coding!!