A rel is a unit of measuring the strength of subspace.
A rating of 1 rel indicates a "calm" area of subspace. A ship traveling at warp in a current of 10 rels would amplify the ship's warp field by a factor of 10, allowing the ship to travel at 10 times the apparent velocity of a ship traveling at an equal warp factor through 'calm' subspace.
A area of space with a subspace strength of 0 rels is a subspace sandbar.
The USS Baldwin encountered a current of 25,000 rels in 2377. At that strength, Baldwin was not able to drop out of warp. Baldwin captain Patrick Ingrum ordered his ship to chart the ship to the end of the current. (USS Baldwin: "Against the Wind")