What is this symbol on the Arduino Uno schematic?
The component is a “ceramic resonator” and the capacitors are internal to the component.

Resonators can be used in place of crystals in applications where the accuracy is not very important. Typically a resonator might have an accuracy of something like +/-1% or 0.5% versus tens of ppm for an inexpensive crystal.

The temperature and aging drifts are also worse. The advantages include less issue with drive level and faster (and more reliable) start-up. They may be cheaper.

The part number on the original schematics reportedly refers to a Murata CSTCE16M0V53-R0, though some clones use a crystal and external load capacitors. That’s a 16MHz +/-0.5% initial tolerance part with 15pF load caps.

Photo from Digikey: