The round shield with a star and stripes is one of my fovourite props. It's made of vibranium, the most durable and the rarest metal in the world, designed by Iron Man's father, Howard Stark.
The shield belongs to Captain America and is tool to protect and to attack. You can see it in a couple of Marvel films, for example "Captain America" or "The Avengers".
In the Marvel universe the shield is very important. Many times it helps Captain America to fight, to save himself and his friend. He relies heavily on it. He wouldn't be Captain America without his shield
The shield is so recognizable, that it appears in other films, like in "Free Guy" - the main character uses it as a weapon in a computer game and everyone gets the hang of this joke.