This is a pictograph of a mat with each short edge split into three units. It therefore denotes a floor or pitch area with a width of three standard mats.
Illustrative Text Reference:
Mohenjo-daro: Seal: M-396 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 96: Collections in India: Suomalainen Tiedeakatemia.
Image Credit:
Indus Script Sign Number 1002: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.