Yom Kippur simply means "Day of Atonement" in English. It is a Jewish word that represents a period after Rosh Hashanah (the head of the year in the Jewish calendar) when people offer prayers and sacrifices to God as they are reflecting on their past mistakes and also turning away from their sinful ways. Yom Kippur is observed on the tenth day after Rosh Hashanah.
It is believed that the period of ten days between Rosh Hashanah and Yom Kippur is for God to judge the past mistakes of people, and on the tenth day, which is Yom Kippur, God will decide on the fate of every single soul. So, to find favor in God's sight, Jews are always advised to offer prayers and sacrifices to God through fasting, with the belief that God will be so kind in making his judgment over them. Yom Kippur lasts for a period of 25 hours. People are not expected to eat or drink anything.