[ Please Help Me, Urgent ] About concurrent proses

Dear All;

I would like to ask information or theory about ruby on rails.

I have question about 2 the proses concurrently that are run together in ruby on rails. But ruby doesnt run those proses concurrently, ruby runs it like a thread. My question if 2 proses are run concurrently in the same time even the same second, what is thing that can make ruby to take decision which proses will be run first then another should wait??

I hope you guys can understand clearly my question. Thank you very much for your suggestion and answer.

Thank You. Rein