In music, a barre chord (also known as bar chord or rarely barr chord) is a type of guitar chord, where one or more fingers are used to press down multiple strings across the guitar fingerboard (like a bar pressing down the strings).
This chording technique enables the guitarist to play a chord that is not restricted by the tones of the guitar's open strings. For instance, if a guitar is tuned to regular concert pitch, with the open strings being E, A, D, G, B, E (from low to high), the only chords one can play while utilising these open strings are based on one of these notes. To play an F♯ chord the guitarist will need to barre strings so that the root of the chord will be F♯.
Barre chords are often referred to as "moveable" chords, as the whole hand may easily be moved up and down the neck, "in one movement".
Commonly used in both popular and classical music, Barre chords are frequently used in combination with "open" chords, where the guitar's open (unfretted) strings are used to construct the chord.
Playing a chord utilising the Barre technique will slightly affect the tone quality of a chord. A closed, or fretted, note has a different sound to a note produced by an open, unfretted, string.
By using the Barre technique the guitarist can fret a familiar chord that is usually played with open strings, and then transpose, or raise, the chord a number of half-steps higher, similar to the use of a capo. For example, if a guitarist plays an E major and wants the next chord to be an F♯ Major, then barring the open E major up two frets (thus two semitones) from the open position will result in a barred F♯ Major chord.
Courtesy : Wikipedia