Does spiritual death mean loss of faith or doing a deadly sin (or sins) or separation from God?

According to Word-of-Faith movement , Jesus suffered both physical and spiritual death. They mean by spiritual death that he suffered in hell after dying, before resurrection. And suffering in hell means separation from God. I ask what is the separation from God?, What does it mean? Hence, I ask: Does spiritual death mean loss of faith or doing mortal sin (or sins) or what? The Term separation from God is unclear to me.