Plain English Meaning
A beacon signal where two quick white flashes appear close together, instead of one. When you see this pattern alternating with green, the airport is a military one, not a civilian airport.
Full Definition
A pattern of two rapid white flashes emitted in quick succession by an airport's rotating beacon, used in addition to the alternating green-and-white flashes to identify a lighted military airport. The dual peaked flashes distinguish military fields from civil land airports, which display a single white flash alternating with green.
Context Anchor
Seen in discussions of airplane exterior lighting, especially anti-collision or strobe lights used to help make an aircraft more visible.
Derivation
"Dual peaked" comes from the shape of the light intensity over time: two distinct peaks of brightness occur close together, rather than one smooth flash. The phrase describes what the eye actually sees -- a quick double of white light.