Naquadah is an extremely useful metal used across the universe. The Stargate is composed of it and most Goa'uld technology utilizes it. The metal is so important that many planets' sole task is to mine it, as was the case on Abydos. The SGC uses naquadah in the form of a reactor to create large amounts of energy in a portable format.
One time the guy who briefly replaced General Hammond tried to make a naquadah-powered nuclear bomb, what an asshole
The micro-thin, titanium (and later titanium-trinium alloy) shield that is used to keep unwanted intruders from entering the SGC via the Stargate. For security purposes, it is generally kept closed until a valid GDO signal is sent through an incoming wormhole. When it is closed, unwanted intruders are unable to fully re-form upon exiting and are killed.
General Hammond ordered the iris closed to prevent a Goa'uld attack.