Of Japanese Descent
noun, の adjective
The 日 is the 日 in 日本, making it mean Japanese in this word. If you are Japanese in lineage then you are of Japanese descent.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
My wife is a third generation American of Japanese descent.