The Object Repository is the collection of property/value pairs that represent an object. This is how QTP will be able to identity the objects and interactive with them. When a user records a test script, the object's property/value pairs are captured at runtime and then stored in the object repository. Then when you play back the test script, QTP tries to match the stored values in the object repository with the new property/value pairs at run time.
You can see all the objects associated with your test on the left hand side when you click on the available keywords tab.
To open the object repository you click on the yellow, green, and red cylinder in the menu.
The object repository will look like this:
As you can see QTP picked two property/value pairs to identify the OK button. text = "OK" and nativeclass = "Button."
You can add or remove property/value pairs, you can set up a visual identifier, you can set up an ordinal identifer, and add additional details for objects in the object repository.