How to run multiple tests in parallel

In my previous company, when we run regressions, the tests are run in parallel which makes the regression finish faster.

There is an already-made script that do that.

Well, I'm not familiar how did they do that so here in my new employer I'm running regression differently which runs the tests one by one, thus it takes so much time to finish the regression.

So, how do you run all/some the tests of the regression in parallel?

Thanks.