Explain ace inhibitor use as a anti hypertensive?

Relaxes vasc muscle: Angiotensin i is a hormone that is converted to angiotensin ii with the help of the angiotensin converting enzyme (ace). Angiotensin ii is a very powerful constrictor of vascular muscle and in doing so increases the pressure inside the vessels (the blood pressure). If you inhibit (or neutralize) the ace then you don't produce as much angiotensin ii and the BP does not increase.

Aldosterone: Aldosterone is a hormone in your body that increases blood pressure (necessary in evolutionary sense). Ace inhibitors block an enzyme that leads to the production of aldosterone. Block the "ace" means less aldosterone which means lower blood pressure.