What does it mean when Scripture says that Christ was the Lamb "slain from the foundation of the world" in Revelations 13:8?

Rob Ventura explains in this video that this verse explains how it was God's plan of salvation. This was God’s plan, that His Son would be slain. In the fullness of time, Christ would die on our behalf.

It was His eternal plan that His Son would be slain for sinners. 

1 Peter 1:20-21 (NKJV) – “He indeed was foreordained before the foundation of the world, but was manifest in these last times for you who through Him believe in God, who raised Him from the dead and gave Him glory, so that your faith and hope are in God.”

So when the Bible speaks about Christ, the Lamb of God being slain from before the foundation of the world, I really view that passage as that which was in the mind of God. Of course, because God's mind, His eternal mind, Christ was going to die for our sins and that's how I view that passage. He was slain from before the foundation of the world in the mind of God. We can say in the plan of redemption That was always going to happen and then in the fullness of time as Paul speaks of in Galatians 4, God sent forth his son born under the law to redeem those who were under the law that we might have the adoption as sons. But this is God's eternal plan that the Son of God would be slain and thanks be to God that in the fullness of time that's exactly what happened on our behalf.