Complex sentence: Yom Kippur (the day of atonement) is a day of fasting at the culmination of which Judgement for that year is sealed, therefore Jews strive their hardest to make certain that they have performed Teshuva before the end of the day.
Split into simpler sentences: Yom Kippur (the day of atonement) is a day of fasting at the culmination of which Judgement for that year is sealed. Therefore, Jews strive their hardest to make certain that they have performed Teshuva before the end of the day.