A post hole showing as a dark circle before excavation at Lawn Farm, 1999

A posthole, as the name suggests, is a hole in which the supporting post of a structure once stood. They tend to show up in archaeological excavations as a circular patch of darker soil.
They are important to archaeology as they can show exactly where a building or structure once stood. On larger sites they can show how the area was once used and can be used to recreate the structure they once housed.