Understanding where the root notes are for each guitar chord you play is quite important.
In general, the root note is going to be the lowest note in your chord, in terms of pitch. Definitely this is the case in all of the standard open chords and bar chords. Later on, you'll get into more advanced chords, and probably see some inversions and slash chords and things.... but don't worry about that for now!
The reason you need to know where the root note is, is so that you don't strum anything lower than that note! If you're constantly just strumming all six strings, your playing will sound muddy and not very precise, or even musical!
Take for example the D major chord. As you can see in the diagram, the lowest two strings are marked with an X indicating that we are not supposed to play them. The fourth string is played open, and then we have the notes on the first, second and third strings.
Let's look at the actual notes in the D major chord. In order, that lowest open note is a D, then we have A, D and F#.
Now, what are the two notes with X on them? E and A.
E has no place in the D major chord. If you play it, it really conflicts with the D, making the sound more muddy and less defined. But what about that A? We do have another A in the chord, so why can't we keep this one too?
The reason is that the lower frequencies really set the stage for how our ears interpret the higher frequencies. Think of the lowest note like a starting point. If you start on an A, but you want to play a D chord, it changes the flavor of the D chord.
Technically, this can work, because A is the fifth of D. However, in this case we would write it as D/A. You would say that as "D over A." This is an example of a slash chord, because there's a slash "/" in the chord, indicating that although we're playing a D chord, the bass note or lowest note is A. You could also say it is a "D with an A bass."
Even in this case though, the ROOT note of the chord is the D, because that's the note that gives the chord its name.
As you can see, understanding where root notes are can really help clear up some confusion around chords, and also clear up your playing too!
So once you've watched this video lesson, and you understand where the guitar root notes on your chords are located, pay attention to that while you're practicing. Take the time to think about it, and make sure you're only strumming notes within each chord that are supposed to be there!