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:

If you look carefully, the crystal and capacitors form one component in the schematic.
It is a resonator, a package where the crystal has integrated capacitors. For that reason the capacitor values are not explicitly given (maybe they're in the datasheet of the resonator).

Blocking capacitors prevent D.C from affecting the performance/operation of the resonator(clock). D.C voltages have a tendency to destroy performance in filters as well. The resonator might operate over a broad range, making specific value capacitors useless in this circuit. Depending on pass bandwidth of the resonator, they probably maintain load impedance at a satisfactory level across the device's operating range.