I'll try and explain it.
In inches, a 10 pitch single start thread, will have 10 threads per inch, or ten complete threads for every inch of run. Simple enough right? A two start 10 pitch threaded rod will have two different threads (five threads per inch each) running together, and a five start will have five separate threads running together, or 2.5 complete threads each inch of run.
As you can see, a five start threaded rod would move the nut one inch with just two and a half turns of the rod. Where as the single start would require ten turns to move the nut one inch. By using a two or five start threaded rod you can get more motion per step than you would with a single start thread. Makes things faster, but at a cost of power. Hope this helps.