Logging helps to understand what your running system is currently doing. Read about the possibilities of the logging framework in Logging.
In the EEROS library you will find a directory with examples. For this example see LoggerTest.cpp.
Open a shell in the build directory of your EEROS library and run
$ ./examples/logger/loggerTest
This examples shows various log messages on different log levels. Study the code to understand how matrices and safety levels can be put into log messages.