The debug log is a tool that traces the progress of various system actions and automations.
It contains a lot of information about operations that come from databases, system processes, Visualforce, APEX calls, etc …
It can be used to check the reasons for action triggering problems. You define which specific user to track, and all the system and automation actions that he triggers will be traced over a given period.
NB: The debug log does not include information from actions triggered by time-based workflows.
The debug log is inactive by default.