AEROQUIP .25 ID PLASTIC COIL

That measurement refers to the inner diameter of the plastic coil, which comes in at a quarter inch. That's the opening through which fluid or whatever you're routing through it actually passes. It's a standard sizing reference used across Aeroquip's coil line, so if you're matching it to a fitting or an existing line, you want the mating component to correspond to that same quarter-inch bore.

Primarily, this Aeroquip plastic coil is used for protection and organization, keeping lines and hoses neatly routed and preventing abrasion.