The main goal of Pilotwings 64 is to complete flight tests. Each test has a specific amount of points to meet for a bronze metal, silver metal, and gold metal. By doing well on a test, further tests will open and additional games will unlock.
By getting silver badges or higher on the A, B, and Pilot rank tests, the extra games Cannonball, Sky Diving, and Jumble Hopper will be unlocked. By getting silver badges or higher on all three of the Beginner tests, the extra game Birdman will be unlocked. Finding warp stars in the Rocket Belt classes will unlock other areas for Birdman; warp stars can only be found during Rocket Belt tests.
Each test has a different objective to complete. You may need to either pass through rings or take a photo of something, as examples.