Direct Phase Digitizer

I want to make a digital FM demodulation and already found the block
diagram but i really confused about Direct Phase Digitizer. The
explanation say that it's a phase detector by using zero-crossing method.
Can some tell me the theory please coz i don't know about zero-crossing.
Thx before!

I put the explanation from my source in case u need it:
DPD circuit uses a high-frequency reference clock to estimate the time
between zero-crossings of the limited IF signal. This estimate is mapped
into an N-bit word theta(t) representing the phase of the limited IF
signal relative to reference clock.