What is the policy of Appeasement?

To "appease" someone is to give in to their demands in
hopes of avoiding further and greater demands and/or in order to avoid having to fight
them.  This is what was involved in the policy of appeasement that was taken by Britain
and France before the start of WWII.

Hitler was very
unhappy with the Treaty of Versailles and he wanted to do various things that would
violate that treaty.  These involved things like rearming Germany and taking territory
that Hitler felt should be German.  The policy of appeasement consisted of allowing him
to do these things in hopes that he would not do anything
worse.

For example, then, when Hitler wanted to take the
part of Czechoslovakia known as the Sudetenland (because ethnic Germans lived there) the
French and British allowed him to do so.  They did this in hopes that that would satisfy
him and a war could be avoided.