From the Comprehensive Rules 4.0:
The twilight pool is an area on the table where twilight tokens are placed. The tokens in the twilight pool represent how dangerous the world is for the fellowship. Glass beads (preferably black) make good twilight tokens, but any convenient tokens will do. Keep a large reserve of twilight tokens handy.
In game text, you will find phrases like “Add ” which means, “Add 1 twilight token to the twilight pool.”
See also start of turn, twilight cost.