I googled a saucer and its a small plate you put your teacup on.

I did some research and the word derives from the French word for sauce, and the little shallow plate/bowl itself traces back to ancient times when people used it to hold condiments like olive oil, like for dipping. I know some Italian restaurants today provide them to hold olive oil to dip bread into.

Later, when people started using tea and coffee cups, the saucer was adopted to put the cup on to catch drops and spills.

Neat!