4.6 billion years ago: This term refers to the age of the Earth itself, based on dating minerals and meteorites. It's like knowing when your kitchen was built before you baked your first cake in it.
3.9 billion years ago: This term represents the estimated time when life first appeared on Earth after its formation. Like waiting for your oven to preheat before you can bake your cake.
Abiogenesis:This is the process by which life arises naturally from non-living matter, such as molecules containing carbon and nitrogen. In our analogy, this would be like combining flour, sugar and eggs to make batter - it's not yet a cake (or 'life'), but it's an important step in getting there.