You may not need to change this setting; when you receive a resource calendar, it will be set to the recommended option. Before changing this setting, first understand how your resource calendar is shared and who is able to book it, as described in the article Create and Share a Resource Calendar.

Auto Accept Invitation Settings

If you would like to control whether the resource is booked automatically, you can do so in the calendar settings. Click the arrow to the right of your resource calendar and choose Calendar Settings.

The item "Auto Accept Invitations" has three options:

• Auto-accept invitations that do not conflict - means that anyone who has rights to book the room will have their appointment accepted automatically, but the room cannot be double-booked by an inviter.  The manager of the resource calendar still has the ability to go into the calendar and reject invitations manually.  This is the recommended, and default, option!
• Automatically add all invitations to this calendar - allows inviters to double-book a resource - that is, no matter what, the room takes the appointment.  The manager can then reject all conflicting invitations.  This makes the resource much harder to manage.

Receive Notifications for a Resource Calendar

If you manage a resource and plan to manage its invitations manually, you should sign up for notifications so you receive an email every time the resource is requested.  You can do so by clicking the arrow to the right of your resource calendar and choosing "Notifications".  

If you request notifications for new invitations, you will receive an email each time that resource is requested.  These emails will include a  "More Details" link which takes you to the event, where you can accept or reject it.