Your main parachute is a mechanical device, therefore, it has an inherent failure mode. In short, it can malfunction. When dealing with a malfunctioning main canopy, it is important to stay aware of your altitude.
Some malfunctions are correctable.
If a correctable malfunction can not be corrected by the decision altitude , you must initiate your reserve activation procedure. The decision altitude for students is 2,500 feet. This is when the needle of your altimeter goes into the red. If you have a malfunction with your main canopy, and are not sure if you can correct it, you must make your decision to initiate your reserve activation procedure by 2,500 feet and execute the procedure by 2,300 feet. This does not mean you have to wait until 2,500 feet to initiate your reserve activation procedure.