Every guitarist needs to know the notes and numbers of the guitar strings. If you're a beginning guitarist, this is one of the very first things you'll want to tackle. As I mention in the video, we instructors always refer to the strings by their notes or numbers, so in order to fully understand what we're talking about, you'll need to have the strings memorized.
From first to last, the order of the strings are:
High E string (1st)
B string (2nd)
G string (3rd)
D string (4th)
A string (5th)
Low E string (6th)
Be sure to memorize the strings to the point where you won't forget them the next morning. You want fundamental stuff like this to stick with you forever. It might also help if you give yourself tests, or even have someone else test you. For example...
Someone else: "What's the note of the 5th string?"
Someone else: "What's the number of the B string?"
...and so on, until you have them completely memorized.