Well I am NOT an expert on threads.
Doc: http://www.ruby-doc.org/core-1.8.6/Thread.html
What we DO know is that they were CRAP on PC with Ruby v1.8.0
They are better, in v1.8.6-p287, likely even better in v1.8.7 branch.
But they are still Green threads, not native threads (at least on PC.)
I'm not sure if Apple tweeked the Ruby editions they install on Macs, and what 'kind' of threads you get, on Apple platforms.
If you want your "thing" to work on older SU versions, the API's thread-like UI.start_timer() block method may be a better choice.
Otherwise... if the rdocs are too techno.. check out all the books on Ruby. Maybe tutorials on the web.
We've avoided them on PC because they were too problematic (... likely because PC Sketchup was running Ruby v1.8.0 for so many versions.)
I never (on my machine,) ran Sketchup with the obsolete initial release of v1.8.0; always something >= v1.8.6-p111
~
If YOU get comfy with Threads on Mac (under Sketchup,) a tutorial may help others out.