This link has been posted before but it is worth repeating. This is the Wikipedia link all about QRP.
See https://en.wikipedia.org/wiki/QRP_operation .
I am a firm believer in QRP. Simple gear can span the globe, although rising noise floors are making it tougher for some, especially in cities and towns. In theory, 5W is just a few S-points down on a 100W rig, but these days the limit is more likely to be man-made noise sources at the receiver especially on LF, MF and the lower HF bands. Up to now, I have been blessed with a low RX noise floor here. I am still lucky.