Test coverage refers to the percentage of browser/OS combinations tested on, with 100% coverage being unrealistic due to infinite possible combinations. While setting a goal for test coverage can provide assurance that an application runs properly within most environments, it's essential to make this goal feasible and contextualize it based on how many platforms are supported by the end users. Factors like platform support, automation of tests, and app bug-proneness should be considered when determining the level of test coverage. A general guideline is testing against at least five platforms, but this number can vary depending on specific circumstances, with some apps requiring more or fewer tests due to their complexity or interaction with hardware.