I wanna know about "optical flow"

Hello , everyone,
I wanna know about what is a optical flow , how it’s work properly and which is the best , can I made on raspberry pi ? , And can we made avoiding system using camera (something like a optical flow used camera)

I know this [https://ardupilot.org/copter/docs/common-optical-flow-sensors-landingpage.html]

I need something new , so I understand more than enough.
Thank you for