What is the meaning of ـُ?

ḍamma ضَمَّة (ḍamma) indicates a short /u/ or its cognates, indicates a long /uː/ if followed by a و (wāw).

a vowel sign of Kashmiri.

pēš a short vowel diacritic, representing a 'short u'. It also indicates a long /uː/ if followed by a و (wâw)

piš a vowel diacritic indicating an 'o' sound.

Source: wiktionary.org