I'll tell you what I know about this (beware maybe it's not as simple as you want). Hope it helps and anyone else feel free to correct me if I was wrong.
Normally, you don't need to set it up yourself. It's a
kind of entry point to startup DHT. Torrent tends to decentralized data transfering, tracker is one of thing make it less decentral (because the necessary having a outside server that track large amount of peers, and manage info between them, send peers list to each of them...). So the way torrent can be trackerless, DHT was implemented. Distributed Hash Table (unique for each existed torrent), torrent client once find it out will be able join the swarm where another peers share the same content. But logically you can't start to get DHT without knowing anything about any other peers that already in that swarm for that given DHT. Then Bootstrap node is there for this purpose.
References
Bittorrent DHT is implemented via a protocol known as Kademlia, which is a special case of theoretical concept of a Distributed hash table.
Exposition
With the Kademlia protocol, when you join the network, you go through a bootstrapping procedure, which absolutely requires that you know, in advance, the IP address and port of at least one node already participating in the DHT network.
You can google or read there -
http://superuser.com/questions/592238/in-simple-terms-how-does-a-bittorrent-client-initially-discover-peers-using-dht
http://forum.utorrent.com/viewtopic.php?pid=419905
I think you need google and read about it yourself because it's unavoidable. Either you read it somewhere else or read it here,... you end up reading in order understanding about DHT Bootstrap. Because such thing is not something naturally understandable nor straightforward.
