The reserve is the backup canopy carried on every sport rig, used if the main canopy fails to open properly or opens in a way that cannot be flown safely.
Why it is treated differently
In the United States a reserve may only be packed and inspected by an FAA-certificated parachute rigger, and must be repacked on a regulated cycle whether or not it has been used. A main canopy, by contrast, can be packed by the jumper.
Reserves are also designed to open reliably and quickly rather than comfortably, and are generally more docile in flight than a main of comparable size.
Using one is a trained procedure
Deciding when to go to the reserve, and the actions involved, are core parts of first-jump training and are practised on the ground before anyone leaves an aircraft. That training comes from an instructor at a drop zone — it is not something to read about and improvise.
These pages explain what things are so the language makes sense. They are not instruction and cannot substitute for it — skydiving is taught in person by rated instructors at a drop zone, and anything involving equipment handling or emergency procedures belongs there rather than on a web page.