This is a 16MHZ Crystal. This is a low cost crystal oscillator with oscillation frequency of 16MHZ. Crystal are normally required to provide clock pulses to your microcontroller or other IC's which require external clock source. An oscillator crystal has two electrically conductive plates, with a slice or tuning fork of quartz crystal sandwiched between them. The crystal oscillator circuit sustains oscillation by taking a voltage signal from the quartz resonator, amplifying it, and feeding it back to the resonator. Quartz has the further advantage that its elastic constants and its size change in such a way that the frequency dependence on temperature can be very low.