What is it?
One can save a map as an image and put it on a web page, but that's not "web mapping," per se. What we mean by web mapping is embedding a "live" map in a web browser. Most of us have seen this with services like Google Maps:
Run a QGIS server. We can send projects made in QGIS to this server and users can view and manipulate maps in web browsers
References and Resources
Morad Ouasti Web Map Service (WMS) With QGIS 3 minute video that shows very simple use of WMS and Google Earth Images
Glossary (and Acronymography)
WMS web mapping service
WMTS web mapping tile service