How to identify the grid composition of a website?

I just started the web development course at uni, and I got to do this for homework.

• Research 3 responsive websites that use different grid compositions • Sketch the full page mobile version (Go beyond what you can see on the mobile screen to the footer) • Number each block • Sketch the the desktop version and number each block • Observe how each block responds to the different screen size

I don't really understand how to identify the grid composition of a website, so if you could explain it to me I'd really appreciate)

Thanks.