An SMD crystal is a quartz piece with two terminals polished and sprayed with a silver film. When an IC is packaged with the quartz piece to form the oscillation circuit, it is an SMD oscillator. Y stands for crystal oscillators.
SMD crystal oscillators provide basic clock signals for the system.
On a component, if you see a combination of numbers and HZ or Z, that is a crystal oscillator.
You can see crystal oscillators from the below image. DIP crystal oscillators have two long pins, and the rest are SMD crystal oscillators. SMD crystal oscillators have two or four pins.