Barre Chords - What is a Barre Chord
Barre chords are useful tools for guitarists. Just by knowing a few shapes a guitarist could play many chords in any key. A guitarist just needs to know one major barre chord shape and they would be able to play a major chord in any key. Same with a minor chord, dominant chord, sus chord, you name it. Just learn the shape and you can play that chord in any key, anywhere’s on the guitar.
The Barre
The barre is a technique where one finger is used to fret many strings on the guitar. It is most commonly used when referring to the 1st finger fretting all the strings on the same fret. When done this way, the barre replaces the use of the nut on the guitar. The following is a barre, done with the 1st finger, located on the 2nd fret.

The diagram above clearly indicates that all the strings should be fretted on the 2nd fret with the 1st finger. When played, each string should ring clearly. This may be hard at first, but it will come. The barre will be easier to fret by rotating your index finger slightly towards the nut. Notice that the barre completely replaces the use of the nut. After applying the barre to the guitar, you still have 3 fingers left to fret other notes to make a chord.
Part II - Your first Barre Chord