Well.. its really hard to explain actually. On the surface level its about him living in france and all the people he's involved with. On the popular level it's known as the book that was banned in america and went to court and made the word "fuck" acceptable in literature (it won the case almost 20 somewhat years after it was published).
It's basically an autobiographic beauty. HST was heavily inspiried (but not nessicarily influenced) by Henry.