org.slf4j/slf4j-simpleorg.slf4j.simpleqos-ch/slf4jslf4j-simple is a logging implementation for slf4j-api.
It prints log message emitted at a level of INFO or above to System.err.
If any dependency you have uses slf4j-api you will get errors at startup about not having a logging implementation.
slf4j-simple is not flexible or "powerful" by any definition but, depending on how you deploy your application, it might be all you need.
You need to have both slf4j-api and slf4j-simple available to your program, then you should see output from logging statements.
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
void main() {
Logger logger = LoggerFactory.getLogger(getClass());
logger.info("Hello World");
}