"Web of trust." 
Basically they just serve up people you follow, people who they follow, and however many levels up it's set to. 
It's a good solution to the spambot that's been stress testing the network the past couple days however it has a fatal flaw. Ie; if you are joining right now everyone that does this will not see your posts. I do think it could be mitigated significantly if some heavy weights on here just went through the #introductions feed and followed the green ones. 

Not to worry though, development happens faster on nostr than anything else I have seen. This will all be sorted out shortly I'm sure of it.