what is the motion parallax? and why isit of interest to psychologists?

Dear Thuyngo, 

Motion Parallax is "a depth cue that results from our motion.  As we
move, objects that are closer to us move farther across our field of
view than do objects that are in the distance." (SOURCE: John H.
Krantz, "Motion Parallax",
<http://psych.hanover.edu/Krantz/MotionParallax/MotionParallax.html>).

See further definitions: 
Google - Define:Motion Parallex
<://www.google.com/search?hl=en&lr=&q=define%3A%22motion+parallax%22&btnG=Search>

York University
<http://www.yorku.ca/eye/Motion%20Parallax.htm> 

Naturally, motion parallex is of interest in psychology, because it
tells us something about perception and how our brain receieves and
understands images.

I hope this answers your question. Please contact me if you need any
further clarification on this answer before you rate it. Search terms:
motion parallex.

I didn't know the expression, but one common example is the impression
one gets when driving past an orchard or plowed field:  the near ends
of the rows of trees or furrows seem to whiz by while those in the
distance don't, although one is driving past them just as fast.

thank