A barre is the use of the index finger to hold down more than one string of a guitar at one fret in a single chord, which is then referred to as a barre chord. The most common examples of barre chords are ones in which the index finger holds down five or six strings (on the same fret) while the other fingers hold basic chord shapes. This provides a way of using the simple open string chords further up the fretboard (like a capo). For example, in an F sharp chord, the index finger barres the second fret, and the other three fingers make an E chord using the second fret as the nut.
A cross-fret barre is a type of barre chord in which the barre covers notes on two different frets. It is a very difficult technique that is mainly used by jazz guitarists.