SO, if one camper had unlimited free donuts and wanted to share with other campers, s/he would open a twitter account, and write "free donuts at wwww.xxx.com" and no one would read it because s/he would have no followers until it developed a bunch of 'donut-eating' campers over a period of time.
But, if you know someone that has a large 'following' and sent that person a note saying "tweet this", and s/he did it. Every one of that 'following' might or would read that note ... and suddenly, the donuts would begin disappearing.
Does that explain how it works, basically.