Could someone explain the implications of Locard's Exchange Principle in relation to evidence?

Interesting question, actually.  "Locard's Exchange
Principle" says that when two things come into contact an exchange is going to happen.
 What this means for evidence and crime scenes is that when a criminal goes into a place
to commit a crime, he/she will both leave something as well as take something way in
terms of evidence as a result of having physical contact with the
scene.

Think of it this way...when he goes to the scene he
is going to potentially leave footprints, hair, skin cells, fingerprints, clothing
fibers, and all sorts of other things.  When he leaves the scene he is going to take
with him things specific to that area: pollen, dirt stuck to shoes, plant material, and
that kind of thing.   It is the detective's job to seek out this type of evidence by
understanding that it is always there, someplace.

Locard,
the great-grandaddy of crime investigation, knew that highly trained individuals would
have a chance to help solve crimes by collecting this evidence.