The word Kiddush is derived from the Hebrew root qadash (קָדַשׁ) which means to set aside for sacred use, sanctification, or to make holy. In Rabbinic Judaism, the Kiddush is a formal prayer recited in conjunction with a cup of wine or grape juice in the home or synagogue. The purpose for the ritual is to sanctify the Sabbath or the festival.
Rabbi Alfred J. Kolatch explains the significance of the Kiddush Cup in his book The Second Jewish Book of Why on page 228:
“It is traditional to fill to overflowing the cup of wine over which Kiddush, Grace After Meals, and Havdala are recited. This is often explained as expression of hope that life’s goodness and bounty will be as abundant as the wine that is being blessed.
A more basic reason is related to the sacrificial system in Temple times. When burnt and peace offerings were made, an entire container of wine was poured onto the altar. After the Temple was destroyed and wine was used in connection with home rituals, a custom developed of filling the cup of wine to its very brim so that some of it would flow over.”
Wine is a symbol of joy in Jewish thinking. As the Kiddush is recited with the overflowing cup, thanksgiving is offered for all the blessings of God.