A more thorough explanation:

wifa (wI-f^ə), n.[Old English] Hist. A mark or sign; especially, a landmark showing exclusive occupation or to prohibit an entry.

• A fence around a property is a wifa that shows exclusive occupation.
• A "no trespassing" sign is a wifa that prohibits entry.

The examples illustrate the definition of wifa as a mark or sign that serves a specific purpose. In the first example, the fence serves as a wifa by marking the boundaries of a property and indicating that it is occupied by someone. In the second example, the "no trespassing" sign serves as a wifa by prohibiting entry and indicating that the area is off-limits to unauthorized individuals.