What does God mean by “The life is in the blood”?

An excellent article that answers this question is at https://www.gotquestions.org/life-is-in-the-blood.html

Another good article is at https://www.mindonjesus.com/blood-of-jesus/

The short answer is that the sin of mankind is an offense to God, which must be paid for in only one way – with the blood of a perfect man.  There is no perfect man, except Jesus Christ – so it is through Jesus only, that we can be saved from God’s requirement of Justice for our sins.