Short answer
The saints intercede for us before God, like friends who ask for something on our behalf.
Full answer
Those who have already been purified and live the beatific vision of God reside in Heaven and are called the elect and holy. Not only can they, but also in our favor, help us by interceding for us before God.
Paragraph 2683 of the Catechism of the Catholic Church states that their intercession is the highest service they provide to God's plan and that we should ask them to intercede for us and also for the entire world.
In the Old Testament, we can find the account in 2 Maccabees 15, 11-16, where a vision worthy of faith is narrated, in which Onias (already deceased), with raised hands, prayed for the entire Jewish people. In Revelation 8, 3-4, John’s vision concerning the prayer of the saints is cited.
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