How Debug logs use in Salesforce?

Use of debug logs to track events that occur in your org. Debug logs are generated when you have active user-based trace flags, when you run Apex tests, and when executed code or API requests include debugging parameters or headers.
To view a debug log, from Setup, enter Debug Logs in the Quick Find box, then select Debug Logs. Then click View next to the debug log that you want to examine. Click Download to download the log as an XML file.