What’s the context?
I think people generally mean something which God does not normally or routinely do, but does in a particular instance.
God could expedite someone’s death, for instance, before they carry out some evil plot. It wouldn’t have to be a spectacular intervention; the person could die from natural causes, but under the circumstances people might say, “God intervened” (with which skeptics would have a field day). Or it might be a spectacular miracle, as in Christ appearing to St. Paul and knocking him off his horse on the road to Damascus. Talk about intervention!
Sometimes people say it (half) humorously, for example, your boss gives you an undeserved raise, and you say with a smirk, “Must have been divine intervention!”