The adjective decent means sufficient or acceptable. The local diner you like to frequent may not serve a four-star breakfast, but it probably has decent food.
Though the concept is a little dated, people of "decent society" are socially correct. They don't break the law, behave rudely, use impolite language, or wear inappropriate clothing. Decent can also mean "nice": Holding the door for a woman with a stroller is the decent thing to do. Lastly, decent can also mean "appropriately clothed" (or just "not naked"). If a stranger is knocking on your door, you're probably not going to answer it until you're decent.