Dust caps protect the male half quick disconnect from dirt, dust and damage from foreign objects. The dust plug slips over the male half quick disconnect when not in use. An o-ring inside the dust plug keeps the dust cap connected to the male half quick disconnect.
The dust cap has a plastic coated wire cable attached to it. One end of the cable connects to the dust cap, the other end is usually attached to the male quick disconnect through a hole in the hex material.