A relic is some object associated with a saint. They're a fragment of the saint's body, something the saint used in their lifetime or an object touched to the saint's body. A part of the saint's body is called a first-class relic. A second-class relic is something the saint used. A third-class relic is some object touched to the saint's body.
God shows His approval of relics and their veneration by working miracles through them. An Old Testament example of this is 4 Kings 13:20–21, which reads, "Eliseus died, and they buried him. ... And some that were burying a man ... cast the body into the sepulcher of Eliseus. And when it had touched the bones of Eliseus, the man came to life and stood upon his feet."
A New Testament example is in Acts 19:11–12 which relates, "God worked more than the usual miracles by the hand of Paul; so that even handkerchiefs and aprons were carried from his body to the sick and the diseases left them and the evil spirits went out of them."
Another example involves Christ's garment. Mark 5:27–30 reads:
Hearing about Jesus, she came up behind Him in the crowd and touched His cloak. For she said, "If I touch but His cloak, I shall be saved." And at once the flow of her blood was dried up, and she felt in her body that she was healed of her affliction. And Jesus, instantly perceiving in Himself that power had gone forth from Him, turned to the crowd, and said, "Who touched my cloak?"
The earliest Christians preserved relics of the saints and venerated them. God draws our attention to particular saints by working miracles through their relics. Christians obtain the prayers of saints whose relics they venerate.
Get to know God's holy ones in Church Militant's Premium show, Pillars of the Church—St. Elias.