From Halopedia, the Halo wiki
The UNSC Silver Star is an award given to UNSC soldiers. The Silver Star is awarded for gallantry and bravery in action against an enemy of the UNSC not justifying a higher award. It may be awarded to any person who, while serving in any capacity with the UNSC military, distinguishes him or herself by extraordinary heroism. It is one of the highest citations, below only a handful of others, such as the UNSC Legion of Honor and the Colonial Cross.
List of known soldiers who have been awarded the UNSC Silver Star