A public performance of music as defined by the US Copyright Law is any music played outside of a close circle of associates (under 10) including friends and family that occurs in any public place.
Music, like any other property, is owned by the copyright owner. They have the right to be compensated for the use of their property. When music is played outside of certain educational exemptions (17 U.S.C. §110(1)) a public performance license is needed.
There are currently five (5) US-based Performing Rights Organizations (PROs), and all of them have affordable blanket licenses that cover their entire catalogue. It is important to purchase all PRO licenses when available as many songs may have artists, writers, and performers in different PROs.
State association licenses can be found on NFHS MyGroups.