A deployment bag, almost always called a d-bag, is the fabric bag that holds the packed canopy inside the container. Its job is to make the canopy leave in a controlled sequence rather than dumping into the airflow all at once.
Why the order matters
An unrestrained canopy hitting the airflow at freefall speed would open violently enough to injure the jumper and damage the equipment. The d-bag holds the canopy closed while the lines pay out first, so the canopy only begins to inflate once it is well clear and moving with the airflow.
This staging is what makes a modern opening take a few seconds rather than happening instantaneously.
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.