It means that an amateur radio operator put some sort of special station on the air. It could be a mountain top, it could be a national park, it could be an island, it could be a Maidenhead grid. Whatever the activation, it's someplace that other hams want to contact, probably to win some sort of special certificate or award.As a relative newbie (on the air 3 months) I see the term "activate" or "activation" or "thanks for activating" used on the air. What exactly does that mean?