Basics of HTML & CSS , Inheritence rules , usage of selectors , media-queries , keyframes , bootstrap , creating and using npm packages , using ExpressJS for creating servers , handlebars for rendering in case of JS disabled pages and jQuery for making AJAX calls in case of JS enabled pages , MySQL for data persistence , Sequelize for ORM (Object Relational Mapping) , building REST APIs ,Socket.IO for real-time messaging, MongoDB and mongoose for using NoSQL database , Authentication using PassportJS , consuming the REST APIs over android app using http client,Using Python scripts on NodeJS and running custom sklearn , tensorflow or keras pre-trained ML/DL model in our backend.
Maybe you are trying to learn backend or server side languages and then you came across this term nodejs. Let me explain it to you in simple terms.
When you write your javascript code and run it in browser, it runs perfectly. But what if you want to run it on server(your computer) and not in some browser. Here comes nodejs. So now you can say that it is a runtime enviornment for your javascript. Just type node index.js in your terminal to run your js file. Actually nodejs uses the same engine as that of Chrome's V8 javascript engine.
Nodejs was written by Ryan Dahl in 2009 and is very much used now-a-days.