As it pertains to Captain America, it refers to the second of his trademark shields. The first shield he used, with a typical shield shape with a flat top and a pointed bottom, was just high quality steel. This shield was used by Captain America during World War 2. This non-indestructible shield was destroyed when The Avengers' mansion was taken over by one of the incarnations of the Masters of Evil.

The more familiar round shield, with its distinctive red-white-and-blue paint pattern, is indestructible. It is made out of a Adamantium-Vibranium alloy, and is one of the few things in the Marvel Universe that is harder than plain old Adamantium. Therefore, it can be used to block Wolverine's Adamantium claws and the edge of the shield can be used to remove the head of Adamantium-shielded evil robots (like Ultron).

With its aerodynamic and round shape, the shield can be used as a thrown weapon. And it tends to bounce off of hard objects. It's effects on things like evil robot necks suggest that the edge is sharp, but Captain America seems to be able to catch it wearing non-special gloves, so it probably isn't that sharp.