APM (Application Performance Monitoring)
Also written as APM, Application Performance Monitoring, New Relic, Dynatrace, AppDynamics, Datadog APM, Elastic APM
Tools that watch a running application from the inside and show where it's slow or failing: which request took too long, which database call caused it, and how many users were affected. New Relic, Dynatrace, AppDynamics and Datadog are the big commercial products. APM is one part of the wider practice of observability.
Think of it like
A fitness tracker for the application, showing not just that it's tired but which muscle is straining.
Junior or senior?
Installing an agent and looking at dashboards is common; using the data to find and fix a real slowdown is the differentiator.
Senior sounds like
Has tuned alerts to cut noise and can describe a specific performance problem they tracked down.
Ask them
“Tell me about a slowdown you found using your APM tool. What did it show you, and what was the fix?”