Being able to answer why a system behaved as it did from the data it already emits — logs, metrics, and traces tied together by a request id.
Monitoring tells you something broke; observability tells you where and why. The difference is an outage measured in minutes instead of an afternoon of guessing.
We propagate a request id across services and into logs, trace the slow paths, and alert on symptoms users feel — error rate and latency — rather than on CPU graphs nobody acts on.