[ 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.