Nippon/Nihon means Japan.
Nippon is the name of Japan called by Japanese people.
Kanji letter combination means origin of sun.
In my theory, the reading ways of kanji letters (=Chinese letters) in Japanese and Chinese are different.
Sound of "Japan" is transformed from Chinese way of reading when ancient time travelled long way from eastern Asia to Europe.
It should be noteworthy that usually peace-loving Japanese would prefer to call their country "Nihon", because during WW2 they were forced to call their homeland "Dai-Nippon-Teikoku" that means Great Imperial Japan.