Logging
The running record a program writes about what it did, line by line. The first place developers look when something breaks.
What it tells you about a developer
Good logging is a habit developers build by debugging production under time pressure. Someone who writes thoughtful logs has been the person hunting through them at a bad moment and decided the next person deserves better. It pairs naturally with monitoring and incident experience.
A log is a timestamped line of text a program writes as it works: a request came in, a payment succeeded, an error occurred. At any real scale, logs from many servers stream into one searchable place, using tools like Splunk, Elasticsearch, or Datadog. Debugging a live problem usually starts with a search through the logs.