Barre Chords are played by barring your index finger across the fretboard to form a moveable chord pattern based off the 6th and 5th string more commonly but sometimes the 4th string is used.
These types of chords are common since they provide a simple reusable chord form that is easy to remember and once learned easy to use and play. They also allow guitarists to play chords up and down the neck using most of the strings for a fuller sound.
To see the difference let's compare an Open position A Major Chord to an G Major Barre Chord with the Root (G), off the 5th Fret of the 6th string:
RIGHT Handed Guitar
LEFT Handed Guitar
This Barre Chord finger pattern can be moved up and down the Fretboard to create other Major Chords whose Root is also on the 6th String, whereas open position chords can only be played near the NUT of the Fretboard. By using other Barre Chord patterns we can also use these moveable chord patterns where the Root is based off the 5th & 4th strings.
To use Barre Chords effectively you will need to know the names of the Fretboard notes for at least the 6th and 5th strings. You only need to learn the note names for the first 12 frets, since note names are the same after that, they just sound an Octave higher in pitch.