What are DeviantART commissions?

Your definition of commissions is correct - it is essentially when an artist is hired to create something for another person. People on DA pay for commissions with either money or points, but speaking on a personal opinion, pay with money if you can. Set up some pieces to use as your portfolio, so people can know what your artwork looks like. And be sure to post in the forums to let people know that you're open for business.