Accidentally knocked a capacitor off a laptop motherboard seemingly related to USB
It's a smoothing capacitor. They are used to smooth out transients in current draw by USB devices plugged in to the port. 

If you plug in a device that uses large peak currents - such as a Wi-Fi dongle or USB HDD - then without the smoothing capacitor the 5V rail of the USB port may drop with any spike in current. Depending on the amount at which it drops you could potentially see USB devices reset if the transient is large enough to lower the voltage below what the device can run at.

If I were you I would replace it - a 220uF 6.3V capacitor is called for by the schematic.