In node.js, how the 'use strict' statement is interpreted?

In node.js, how the 'use strict' statement is interpreted?

I have started to explore the Node.js and wrote many demo webapp, to
understand the flow of node, express, jade, etc..
But one thing I came across recently, is the statement "use strict" as
first line inside every function and every file.
So, my question is after doing lot of search, I am not so clear how
exactly it is interpreted by node.js. Please could anyone briefly tell me
about it?
Disclaimer: I am very new to stackoverflow and learning node.js still..