I dont know what exactly reffering to. Anyone can explaining to me?
EDIT: this notation appear on "450 Noëls" by Rouher, Edouard Marcel Victor
The piece is for (pipe) organ. Organs generally have more than one keyboard - one or more for the hands (manuals) and one for the feet. See for example this page at organtutor.byu.edu. These different keyboards are linked to various stops. Different stops control different sounds. The selection of stops (and assignment of particular notes to particular keyboards) is often left to the performer, and is known as registration.
In the scores you are looking at, someone - either composer or editor - has specified particular assignments of notes to keyboards. You can see in the pages before the scores this sort of thing:
Note the G, P, R on the left.
Then, in a score, for example #221:
The R indicates that all the braced voices are to be played on the keyboard that R refers to (Swell, in English)
Sometimes the indicated keyboard changes midpiece:
Here, the instruction is to use Swell for the top two voices of the piano section, but (optionally) switch to Great-Organ or Choir for the forte section.