The Arabic word hilal means the crescent moon used to mark Ramadan’s start. In Islamic calendrical practice, the relevant crescent is the first visible sliver after the astronomical new moon, not the full moon or an ordinary crescent symbol.
Ramadan begins with the ninth month of the Islamic lunar calendar. Because lunar months last 29 or 30 days, communities traditionally look for the hilal after sunset on the 29th day of Sha'ban. A confirmed sighting begins Ramadan; if it cannot be seen, Sha'ban is completed with 30 days. Some modern authorities instead rely partly or entirely on astronomical calculations, so start dates can differ between countries and communities.
Hilal is often confused with crescent-shaped Islamic imagery generally, but the word specifically refers to the visible lunar crescent. Kiswa is the cloth covering the Kaaba, salah means prayer, and qibla is the direction of prayer. A related fact readers often ask about is that Ramadan ends when the next month, Shawwal, begins after its own crescent determination.