In an organ, "Swell" (German: "Schwellwerk;" French: "Récit") refers to the division whose pipes are enclosed in a swell box. This box has a large opening covered with moveable shades or shutters which resemble heavy venetian blinds. When open, these shutters allow the pipes' sounds to travel freely from the box to the room. When closed, most of the sound is contained in the box. Thus the moveable shades provide a means of adjusting the loudness of the sound, and perhaps more importantly, of gradual crescendo ("swelling") and decrescendo.
The reason this arrangement is necessary is that a given pipe only plays at one given loudness. If the wind pressure were varied in an attempt to change the loudness, the pitch, tone quality, attack and decay and other characteristics would also change. In fact, organbuilders have to go to a lot of trouble to provide a steady, unchanging wind supply. So the only way to gradually increase or decrease the loudness of a pipe is to enclose it in a swell box, and then to gradually open or close the shutters.
This has its drawbacks. No matter how a swell box is designed, the sound of the pipes is compromised by enclosing them. Since much of the best organ music was written before gradual crescendo and decrescendo effects became common, this would seem to represent an unfortunate and unnecessary degradation of the music. On the other hand, building an organ with no swell box - and thus unable to play later music - would also seem to be an unfortunate compromise.
The usual way of dealing with this problem is to build an organ in which the pipes are divided into several sections or divisions, one or more of which are enclosed in a swell box or boxes, the other divisions remaining unenclosed. Fortunately, the kinds of music which are least compatible with enclosed pipes are precisely the kinds where gradual crescendo and dimenuendo are not required.