What is the “出生証明書のアポスティーユ Apostille for the birth certificate” when getting married in japan?

I am at a loss for what this document is supposed to be and how I am supposed to get it. Very confused atm. I have just ordered my birth certificate online from the state I was born in (USA) and they will only issue the birth certificate to me directly in Japan. But then I need to send the birth certificate back, from Japan to the secretary of state of my birth state for them to give me this Apostille document???

I have never heard of this before and am curious if any other americans have gotten married in Shibuya recently…