I'll answer by quoting Hitler's own words, written in Mein Kampf (1925):
"I decided upon ... a flag of red material with a white disc bearing in its center a black swastika.... The new flag appeared in public in the midsummer of 1920. It suited our movement admirably, both being new and young. Not a soul had seen this flag before; its effect at that time was something akin to that of a blazing torch. ... We National Socialists regarded our flag as being the embodiment of our party program. The red expressed the social thought underlying the movement. White the national thought. And the swastika signified the mission allotted to us - the struggle for the victory of Aryan mankind and at the same time the triumph of the ideal of creative work which is in itself and always will be anti-Semitic."