That's a question that still eludes any scientific/physical/material explanation. Life is just the opposite of dead. But what animates living things, what life is in that it animates living beings, when it's gone it's gone, yet when it's there there's nothing that can be detected beyond the behaviors of the organism. A live human body and a deceased human body have the same number of particles. Structurally there's no difference.
This is what baffles me by people who look at natural processes and causes as somehow meaning there's no God or no need for a God. They leave this whole element out. Understanding biology and the body medically is just understanding it structurally. Understanding how it physically works. But being able to sustain or maintain life doesn't mean there's an understanding of life. Only what's physically needed to sustain it.