A coup is when someone removes the government and makes himself ruler. The one who coups, that is, the putschist, does not have the right to do so. Another word is coup d'état.
The coup plotters are often politicians or leaders of the army. They think that the previous government is not working well. They say that the mistakes of the old government are bad for the people. Or the government was not elected democratically at all.
That is why the coup plotters plan to remove the government by force. They have the members of the government and other people arrested. Sometimes the coup plotters kill them. Then the putschists form a new government according to their own wishes.
Sometimes a government coups itself. It is not allowed to do everything it wants: laws are decided by the parliament. If the government fights the parliament and prevents it from working, then that is also a coup.
Usually people think a coup is bad. But it can also be that people think badly of the old government. In that case, people may think the coup is right. In any case, a coup is forbidden: It is called high treason, a betrayal against the highest power in the state.