Lent is the Church's longest penitential season — forty days (not counting Sundays) of fasting, prayer, and self-examination leading to the celebration of the resurrection at Easter. By the fourth century it had taken a recognizable shape across the Church.
The number forty is deliberate. It recalls Christ's forty days of fasting and testing in the wilderness, and behind that the forty days of Moses on the mountain and the forty years of Israel in the desert. Lent is the Church walking, for a season, that same wilderness road.
How Lent is kept varies widely — strict fasting in the Eastern Orthodox tradition, the giving-up of particular things in the West, a renewed discipline of prayer and almsgiving across the traditions, and in some Protestant churches a quieter observance. The shared aim is older than any of the variations: a return, by way of the desert, to the joy of Easter.