Q: What is bug tracking system?
A: It is about track an issue in system. In other words, suppose there is a bug which is found during verification process and then it is reported by testing team/tester to developer by using any bug tracking tool or there may be a documentation process.
Process can be followed with one of bug tracking system. In market many bug tracking management services are available. Some system provide complete management flow from test cases creation to bug tracking management.
Taking an example to know about track an issue, like a module is developed and tester get an issue during verification then tester report issue on bug tracking system now senior will be able to track issue status means when issue did report by tester and when fixed by developer if not fixed by developer or not verified by tester whereas fixed by developer senior can track the status.