This is the noun summer, which is preceded by a glyph that marks the beginning of an area, and followed by a glyph that marks the end of an area. Hence, the symbol is an ideograph for the noun
summertime or summer season.
Illustrative Text Reference:
Harappa: Seal: M-1715 A (There is no view a): Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 239: New material, untraced
objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia.
There is only one extant example of this Indus sign.