A reservation is something set aside, like a table at a restaurant or land for Native Americans.
For fancy restaurants and hotels, you need a reservation: you call ahead and ask them to set aside a table or room for you. A reservation is also a piece of land set aside by the government for a group. In the U.S., there are Native American reservations. Sometimes a reservation is a doubt. If you have reservations about taking a trip to Australia, then you're not sure about it. You're not rushing to make reservations at a hotel there.