A WebSDR is a Software-Defined Radio receiver connected to the internet, allowing many listeners to listen and tune it simultaneously. SDR technology makes it possible that all listeners tune independently, and thus listen to different signals; this is in contrast to the many classical receivers that are already available via the internet.
AC6V.com is a massive website with over 700 amateur radio topics, 6,000 links, 133 pages from antennas to zones. Seriously, there is a TON of content on this site, be sure to search on it for what you’re looking for.