I finished reading a book today called "He Loves You"

It was not a bad read.   It was about grace yet it debunked the atonement theory.  I will discuss this in another thread.

So, in short the atonement theory is that Jesus died for our sins in place of us who deserved to die.  Jesus is the Son of God who came to earth only to die so that we could go to Heaven.  He was the sacrificial lamb.  The last one we need.  Jesus committed no sin so He was qualified to die in everyone's place.