The term "top brass" is a noun. For example, "Today he will go before a review board comprised of the top brass." In this case, the decision makers will conduct his review.
The term "top brass" refers to the most important individuals in an organization; military, business, political, or otherwise. For example, "The top brass will be reviewing the parade from the grandstand." In this case, the General Officers will stand and review the soldiers from the grandstand as they pass by.
The term "top brass" can also be applied exclusively to the decision makers, or most important members of any governing body. For example, "Whether the ordinance stays in effect will be up to the top brass." In this case, the city commissioners are deciding on whether to continue a local ordinance.
The term "top brass" can be used to refer to higher ups in other organizations as well, like the church. For example, " The Catholic Church will call together their top brass to address the problem."
The term "top brass" also denotes something of very high quality that is considered "top drawer" or the very best. Many businesses have chosen the term "top brass" as part of their name so that customers associate their name with quality.