Motion vectors and motion estimation

[nickNip]

I want to compare 2 or more frames in video and need to extract redundant objects from that frames.I'm going to use block matching algorithm for this.But I'm not clear about what are motion vectors,motion estimation ,And how it will help to find redundant objects in video frames.I'm new to signal processing field can someone explain this in simple manner

[snibgo]

Ffmpeg has tools for this, helped by the fact that compressed videos already has this information encoded.

IM is useful when the camera is stationary, ideally without vegetation in the scene. Then a pixel-by-pixel comparison finds moving objects, making a small allowance for changing light.