Observability
Following one user request as it passes through many services, timing each step, to see where it slowed down or failed.
Formal
A method where each request gets a shared ID that is passed from service to service, and every service records a timed step, called a span, under that ID, so the full path can be put back together as a trace.
In plain English
Like the tracking page for a parcel, showing each depot it passed and how long it sat there, so you can see it was stuck three days in one place.
In practice
A trace of a slow page in a region's patient portal shows the web service answered almost at once but waited four seconds for the appointment booking service, so the team knows where to look.
Why it matters
When one click touches dozens of services, logs from each one alone cannot show the chain of cause; traces link them into one story, which also helps follow an attacker's steps.