I am a total newbie - so please be kind and patient

I'v been told that the most productive (not the only) way to start a topbar hive is to get a swarm because they are most productive.
Why did the swarm start to begin with? Is a swarm created because a queen is not productive? Or because more room was needed? How do you know, since you wouldn't want a less productive bee.
Any insight, links, truly appreciated.
Colleen