What is the meaning of Hamza?

A sign used in the written Arabic language representing a glottal stop. Hamza may appear as a stand-alone letter (ء (ʔ)‎) or most commonly diacritically over or under other letters, e.g. أ (ʔ)‎ (over an alif [ا‎]), إ (ʔ)‎ (under an alif), ؤ (ʔ)‎ (over a wāw [و‎]) or ئ (ʔ)‎ (over a dotless yāʾ [ى‎]). The exact seat of hamza is governed by an orthographic rule – the "seat of hamza rule".

hamza (sign representing a glottal stop)

Source: wiktionary.org