The capacitance a crystal needs to see to oscillate on-frequency. Wrong caps pull the frequency; matters for USB timing budgets.
How it looks
Drawing: CORE3588 has no crystal of its own; the ESP32 module carries one inside
The two amber capacitors set what the crystal "sees".
How it works
A crystal is a sliver of quartz that rings at a precise frequency, giving a chip its clock.
It rings exactly on frequency only when it sees the capacitance its datasheet names (the load capacitance, CL). Two small capacitors plus the stray capacitance of the board provide it.
Wrong values pull the frequency off slightly. For USB, whose timing budget is tight, that can break the link.
Where it fits
Sits next to a MCU or SoC; the clock is multiplied up by a PLL.