After the final "Alleluia" is sung, on the last Sunday before Ash Wednesday or in some places after Mass on Fat Tuesday, a ceremonial burying takes place. A banner or scroll or object with the word "Alleluia" printed on it is placed in a small casket or box. Then it is solemnly processed to the churchyard and buried in the ground.
I read of one mother who, in the days before Lent begins, stitches the word "Alleluia" on a small piece of cloth. Then she stows it away until the family Easter egg hunt. The child who, on Easter Sunday, finds the egg with the "Alleluia" in it receives a special prize.
"Alleluia" is a Hebrew word which means "praise Yahweh!" It is a word to be sung with unrestrained triumph and exhilaration. So it makes sense to retire or let that word lie dormant for forty days as we set aside the feasting and merry-making for the humbling and penance of Lent.
Outside of Lent we proclaim "Alleluia" every single day in the liturgy. As part of your Lenten journey maybe you could think of one of your favorite words that you use almost every day. Then vow not to speak that word for forty days. You surely will notice how much mindfulness that requires and how your inner life must adjust.
That's what Lent is about. Be truly mindful and may your inner life be adjusted well!