Sudetenland is a region in Czechoslovakia whereby the area is occupied by Germans. The area was part of Austria previously and certain sections were predominantly German.
The annexation of the sudetenland was a claim that Hitler believed belonged to a greater part of Germany. It had a population of over 800 000 people. The French and British did not want war and rather appease Hitler by granting the annexation of the sudetenland.
The Munich agreement confirmed that and was informed to Czechoslovakia that you can either fight or succumb to the Germans. But as long as the French and British got assurances that Hitler does not want war.