This is a pictograph of a perimeter wall with a track going through one side. It depicts an entrance or gateway in the wall. Moreover, because the location of the cardinal points is governed by the Indus convention, whereby north-west is located in the centre at the top of the character area, we can determine that this symbol means east gateway.
Illustrative Text Reference:
Harappa: Tablet in bas-relief: H-1988 A: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 272: New material, untraced objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia.
Image Credit:
Indus Script Sign Number 263: Sign List of the Indus Script: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.