You know how rock bands are always forming and then breaking up? When they break up, they disband. The word disband refers to any group or unit of folks who decide to go their separate ways.
Long before there were rock bands, groups of friends or warriors would band together for protection or companionship. They would become a band of sorts. The prefix dis comes from the Latin word for "apart." So if people come together to form a band — like Robin Hood's Merry Men — then if they disband, they decide to separate and break apart. Groups don't always disband because they want to. Sometimes police officers or government officials disband groups they believe are a nuisance or a threat.