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).

Forms the nominative determinative inflections of nouns.

Forms the indicative mood of present-tense verbs.

a vowel sign of Kashmiri.

a vowel diacritic in Persian, named ضمه (zamma / zamme) or پیش (pēš / piš).

Source: wiktionary.org