These guys are a group of Christian missionaries who are travelling throughout the area, spreading the word about Jesus (3 John 1:7). They're friends of the elder and they stay with Gaius when they're in his hometown (3 John 1:5). Oh, and they also get shunned by Diotrephes (3 John 1:10).
The elder points out that these folks are the good kind of travelling preachers. (Unlike certain antichrists he won't mention.) Other true Christians should support their mission and their efforts (3 John 1:8)…preferably with food and money.
These guys don't take handouts from nonbelievers (3 John 1:7) because, apparently, non-believers have cooties.