The word reservation has two main meanings:

1. A feeling of doubt or hesitation about something. When you have reservations about a plan, you're not completely sure it's a good idea. Maybe your friend suggests climbing to the top of a tall tree, and you feel some reservations because it looks dangerous. Or perhaps your teacher proposes a new seating arrangement, and you have reservations because you worry about sitting far from your friends. Having reservations doesn't mean you're completely opposed to something; it means you're cautious and want to think it through carefully before agreeing.
2. An arrangement made in advance to hold something for you, like a table at a restaurant or a seat on an airplane. When your family makes a reservation at a restaurant, they're asking the restaurant to save a table at a specific time. Hotels, campsites, and rental cars all work with reservations too. Without a reservation, you might arrive and find no space available.

The word can also refer to land set aside by the government for Native American tribes, called a reservation. These are areas where tribal nations have certain rights to govern themselves and preserve their cultures and traditions.