The trees use that resin as protection against various pests and animals that might harm them. Say it’s 99 million years ago, and an ancient ant is crawling up a tree and becomes stuck in the resin. The tree’s trap has worked; the ant is done for. Soon, more resin covers the poor creature and solidifies. Normally, this resin will be disintegrated by wind, rain, sunlight, or bacteria over time—crumbling along with Monsieur Ant inside. But every once in a while, the resin is protected and preserved so that over many millions of years it fossilizes into amber.
