Your camera lens is made of several pieces of glass. The glass is designed to admit as much light as possible on to the sensor, but at each layer, a small amount is reflected.

If the source light is bright enough, especially in comparison to the background, these reflections bounce around a bunch and eventually hit the sensor. Normally, any reflections are very faint and get washed out by the rest of the image, but the sun is like 1000x brighter than the sky, so even a weak reflection shows up.

I saw that on the ground during a solar eclipse under a tree

Here's another neat one:

Ooh, that's a classic! An inevitable happy accident on your behalf.
Shadows under tree canopies also create the same effect, a lot of little "pinhole cameras" between leaves.

Lens flare

Cool!

What's with all the eclipse posts on the socials today? Did I miss something?