Re: Please, in laymens terms, explain the fourth dimension theory/idea?

Your question refers to two ideas, which I'll answer separately.

a)  In Einstein's theory of relativity, time is added as the "fourth
dimension" that couples with space (3-D) to form the idea of the space-time
continuum.  Before Einstein, the way we thought of time was different; the 
best way I could think to describe the "old" thinking is that time and space
are essentially independent of each other:  things happen, and we just evolve
through time, plain and simple.  You can then think of the "new", Einsteinian
thinking as that of time and space being intimately related.  The "space time
interval" (I don't want to bring in any math, but this is just the difference
of the squares of space and time) between two events is a constant for any 
observer, whether they're on the earth or traveling on a spaceship near the
speed of light.  An object that travels really really fast 
experiences a shorter time interval than an object moving slowly-an example 
is the twin paradox (you can find more reading about it in a modern physics
text).  As human beings on earth, we don't have to deal with this on a daily 
basis.  However, if the idea of space travel near light speed becomes 
possible, we will have to.  In any case, no one can really see it, not even
physicists!

b)  The fourth space dimension.  Think of this:  we are stuck on the 
earth's surface because of gravity, so you can think of our movement on the 
earth as being two-dimensional.  Now imagine being on a mission in a space 
shuttle where there is no gravity:  now you have a third dimension to play
with!  Well, if you can now imagine going from three to four, that's what 
the fourth spatial dimension is about.  Once again, it blows the mind.

A good book to browse over is "The Dancing Wu Li Masters", by Gary Zukav.
The author, not a physicist, writes about various modern physics subjects 
in layman's terms.  It is very much a book dedicated to making physics
understandable - an art that many physicists (scientists in general) do 
not have.