Plain English Meaning
It describes how the magnetic compass in an aircraft is built: a small sealed bowl of clear fluid containing a floating disc with magnets attached. The disc has compass headings printed on it, and you read your heading by looking at where a fixed line on the case lines up with the numbers on the floating disc.
Full Definition
The physical design of the aircraft magnetic compass, consisting of a sealed bowl filled with acid-free white kerosene, inside which a float assembly carrying two small magnets is suspended on a hardened steel pivot point. A graduated card marked with headings (in 5-degree increments, with letters N, E, S, W for the cardinal directions) is attached to the float. The card is viewed through a glass window against a fixed called the . The liquid damps of the float and supports its to reduce on the , while a or allows the liquid to expand and contract with changes.