To certify is to provide proof for something or to license someone, like a lawyer. You can certify a lawyer or certify that someone’s insane. Either way, when you certify something, you make it official.
To certify is to have evidence that something is true, to make certain. If you certify that you own a house, you prove it's yours. Vouching for a friend can certify that the friend is a good person. Professionals like lawyers, doctors, and accountants must be certified. Being certified assures others that the person is qualified. Certify also refers to declaring someone legally insane, so make sure your doctor is certified by the medical board, not the loony bin.