A boss is an authority figure, often the person who tells you what to do at work. If you work as a waiter, your boss might be the restaurant manager.
A boss at a job might hire you, give you feedback on your work, or tell you what your responsibilities are. If someone is in charge of a group, you could describe that person as a boss as well: "She's the boss of the French club — she tells everyone what to do." To boss someone is to do just this, order them around. Boss comes from a Dutch root, baas, "master."