Appeasement refers to the policy, mainly put in place by British Prime Minister Neville Chamberlain but also by other European leaders, of letting Hitler expand the German boundaries.
The generally thinking was that Hitler would eventually be satisfied and would cease to continue his march.
When Hitler took Czechoslovakia and Poland, European leaders did not respond to stop him.
The problem with this policy was that Hitler was rapacious and always wanted more with visions of ruling the world.