Thank you Mr Abdel-Nasser for your answer 

for more technical solution you can read this paper

Werlberger, Manuel, et al. "Anisotropic Huber-L1 Optical Flow." BMVC. Vol. 1. No. 2. 2009.

a non technical viewpount is cognitive/telling, with two or more images you can establish a story.

Hi Dear

A useful definition could be found in the following link

https://en.wikipedia.org/wiki/Optical_flow

thanks

Dear  Rachida :

From an engineering point of view, you can imagine the image flow in the same way as water flow. The water flow from the high land to the low land. In a similar way, the pixel intensity flow from high values to low values. This flow can be positive, negative, or zero if there is no change in the pixel values between images.

Regards,

Gamal