Mechanism to further specify the log levels emitted by the logger.
That means, while your program can continue to have code to emit messages at any level, only levels matching the settings (interval of minimum/maximum, exact level) will produce output.
Imagine you have a production application, it will typically log minimally - for example just info level. But then user reports issue, and you can have ready a command line switch, which will turn the logging on for all levels, to help identify possible issue.
The available levels are: