In the simplest form his book ‘Mein Kampf’ was about Hitler and his ideas and opinions about politics and race. It basically has two parts. Part one was published in 1925 ‘Die Abrechnung’ and the other in 1927 ‘Die Nationalsozialistische Bewegung’.

He wrote the first part while in prison after the failed Beer Hall Putsch. It talks of how the Jews had betrayed Germany and the blame for the loss of WW I. He also talks of the Aryan race was superior with exceptional intelligence and creative power and the Jews where more like parasites bleeding the Aryans. He also talked about how the Germans should expand east ‘Lebensraum’ through the Slavs, another inferior race and take out those darn Marxist and Bolsheviks in Russia. Hitler said it was a sacred mission of the German people to do so. Kind of like a Crusade I guess.

Johanna Steinbrecher's answer to Why did Hitler blame the Jews for their loss in WWI?

Part two was more of how the national political movement will go after power and achieve this goal for a New Germany.

So to put it into one sentence, ‘Mein Kampf’ was about political enemies and race enemies in Hitler’s mind.