A search node is something used by giFT, the backend of Apollon. Although I've never used Apollon myself, I have used other front ends to giFT, and a search node pretty much tells everyone on the network that they can search other computers from your computer. The more search nodes there are in a network such as OpenFT or Gnutella, the faster/more results someone will get. So if you leave your computer on all the time this would be a good option to enable. The downside is that it takes up some processing time (to my understanding, I may be wrong). You shouldn't notice it unless you're running intensive apps such as games or a large compile. I hope this helps!