If we take this idiom literally we could find ourselves in a whole lot of trouble with the police, as it would mean taking a knife or another sharp object and putting into a person’s back.
However, as an idiom to stab someone in the back means to hurt someone who was close to us and trusted us by betraying them secretly and breaking their trust. We call the person who does this a back stabber.
“Did you hear that Sarah stabbed Kate in the back last week?”
“No! I thought they were best friends, what did she do?”
“She told their boss that Kate wasn’t interested in a promotion at work and Sarah got it instead.”
“Wow, that’s the ultimate betrayal! No wonder they’re not friends anymore.”