What is a barre chord?
A barre chord is a chord voicing where one or more fingers plays across multiple strings on the same fret.
Barre chords typically don't include open strings but they can. This makes it easy play certain chord shapes all over the fretboard since you aren't limited to what open strings are included.
For example, you can play the following Em chord with the open E string.
Or you can play the same shape down 2 frets as a Dm chord. For this chord you wouldn't want to play the low E string as it would make the Dm chord sound muddy.
Bm and F are the two barre chords worship guitarists run into most often, and the two most commonly avoided in favor of a substitute shape. However, WGA has tons of alternative chord shapes you can play if full barre chords are difficult for you right now.