I don't understand what it means (and that page doesn't explain) for a 'constraint' not to be enforced? Do you somehow get some sort of warning, but not an erro

I don't understand what it means (and that page doesn't explain) for a 'constraint' not to be enforced? Do you somehow get some sort of warning, but not an error, i.e. the insert/update is allowed to work, everything proceeds as normal, but it's there to check on and correct if you want?