To understand the basic idea of radiometric dating, consider Uranium-Lead dating of Zircon, a basically impregnable crystalline compound of Zirconium Silicate that forms at very high temperatures. Zircon sometimes incorporates Uranium molecules into its crystalline structure as it grows because the configuration of outer electrons are similar. This results in Zircon samples with Lead (which Zircon strongly rejects) unnaturally bound to the crystalline structure because Uranium decays into Lead at a steady rate not changed in the millions of years that it has taken for light to reach us from distant stars where this same process of decay is playing out. The age of individual samples of Zircon can therefore be calculated from the ratio of Lead to Uranium. Though the estimated age of terrestrial samples varies because not all zircon crystals were formed at the same time, the Zircon found inside meteorites is all roughly 5 billion years old. The earth could be younger than that, but other evidence indicates otherwise.
