Landing 300 rockets means SpaceX has preserved 2,700 Merlin rocket engines. In round numbers, the dry mass of a Falcon 9 first stage is about 50 metric tons, so the landing of all these rockets has prevented 15,000 metric tons of metal and other materials from being dumped into the oceans—the equivalent, in mass, of about 100 residential homes.
After seeing that so many people agreed with you maybe I could have said things more explicitly.
If I have but one note, if you’re not sure what someone means don’t try to extrapolate and just ask.