One will often encounter Roman numerals when reading a song transcription. Usually, at the beginning of a song, the chords, the key, and the tempo will be displayed. In some instances, the chord fingering and position will be exhibited and a Roman numeral may accompany a chord. The Roman numeral (such as B VII), means a B Major Chord is to be played on the seventh fret of the low E string, rather than in the first position of the second fret of the A string.
In-position chords are those that occur in a “natural” position on the fretboard. For instance, an F Major is played on the second fret, with the bass note on the low E string (2-4-4-3-2-2). Even if the base note is displaced to the D string (x- x-4-3-2-2), the chord is still in its most “natural” position and contains all of the relevant intervals.
Barre chords are simply chords played on the fretboard, usually without open strings. In other words, the same fingering for an E Major (0-2-2-3-0-0) can be moved up the fretboard to the second fret. Instead of the open E and B strings, they are barred and the result is an FMajor (2-4-4- 3-2-2). If you were to move this formation up the eighth fret, the result would be a C Major (8-10-10-9-8-8).
They may occur anywhere up or down the fretboard, so long as the prescribed fingering remains the same. These chords are named for the bass note, so whichever note is the lowest is the name of that particular chord. For example, if you were to bar the A, D, G, and B strings on the third fret, that chord would be a C9sus4; if you were to bar the sixth fret it would be a D#9sus4 (or an E flat sus4 depending on the key of the song).
These chords make playing an E or A Major possible while remaining in natural tuning (e.g. E x-6-8-8-8-6 and A- 4-6-6-5-4-4). Otherwise, these chords could not be played unless the tune is altered or a capo is used.