This is the adjective high with the addition of some horizontal lines, which represent terracing. Hence, it is an ideograph for an agricultural terrace.
Illustrative Text Reference:
Mohenjo-daro: Seal: M-101 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 38: Collections in India: Suomalainen Tiedeakatemia.
Image Credit:
Indus Script Sign Number 1022: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.