Dispersion

White light isn't one color, it's every visible wavelength traveling together, from red (longer wavelength) to violet (shorter wavelength). When light crosses from air into glass or water, it slows down and bends, and the amount it bends depends on the index of refraction. That index isn't quite the same for every wavelength: violet light bends a little more than red light does in the same material. Send white light through a prism and each color refracts at a slightly different angle, so they fan out into a visible band, violet bending most, red bending least. The prism never manufactured any color. It only sorted colors that were already riding together inside the white light.

A rainbow runs on the same physics, with a mirror step added. Sunlight enters a raindrop and refracts, splitting slightly by color. It hits the back inside surface of the drop and reflects once, then refracts again on the way out, splitting a bit more. The result: each color exits the drop at its own angle, with red appearing highest in the sky and violet lowest, since you're sampling light from millions of drops at slightly different heights.

A fainter secondary rainbow sometimes appears above the primary one, with colors flipped, red on the bottom this time. That light reflects twice inside the drop instead of once. The second reflection flips the exit geometry, reversing the color order, and each reflection leaks out some light, so less survives to reach your eye, which is why the secondary bow is dimmer.