The appeasement policy, in the international sphere, consits on making political or material concessions to an aggressive country or organization in order to avoid starting a conflict (war).
European Allied powers (UK and France), during the 1930s, made territorial concessions to Hitler as, in the aftermath of WWI, they wanted to avoid a new war by all means. After signing the Munich Agreement, also known as Munich Betrayal, in 1938, Hitler managed to obtain the Sudetenland in Czechoslovakia with no opposition from the allies. France even had a prior alliance with Czechoslovakia and dishonoured it when embracing this agreement.
Of course Hitler was not willing to stop with this annexation, and in the end the Allied powers had to declare war as he continued expanding, but before they had allowed Hitler to became stronger with no opposition.
