What are examples of computation heavy tasks for a web app?

I am learning Node js. During my research I have continuously read that Node.js is not good at computation heavy tasks. What does that mean exactly? What are examples that can be considered " computation heavy " for a web application. I wanted to build a e commerce / social networking web app.

Node.js server is single-threaded, meaning that any operation that takes a long time to execute will block the rest of your program from running. This means single thread can only execute one instruction at a time, blocking others.