Cut off from the people
Here’s what it means
To be removed from the covenant community by death, divine judgment, or exclusion.
Why the Bible says it this way
The precise penalty varies by passage, so it should not always be reduced to one legal mechanism.
An idiom is a familiar phrase whose meaning comes from the whole expression, not from reading each word by itself.
A helpful detail: Interpreters debate whether particular occurrences mean execution, premature divine death, or exclusion.