A written description of a software problem that lets anyone reproduce and fix it: the steps, what was expected, what actually happened, with evidence.
A good bug report answers five questions: what is the problem, in one sentence? Which steps trigger it, in order? What should have happened? What actually happened? And on which device and browser? It comes with evidence such as a screenshot, and a severity: does it block sales, or is it a cosmetic flaw?
'The site is broken' cannot be fixed, while 'the pay button does not respond on mobile after choosing delivery' is fixed in minutes. The quality of the report decides the speed of the fix, whether the fixer is a developer or an AI agent. Reports from Fanann's QA Engineer come with screenshots and severity, and there is a ready template in the guide on writing a bug report. Practical tip: put one bug in each report, and run the steps once more before sending, to be sure they really trigger the problem.