Indulgentiarum Doctrina is a set of teachings by the Catholic Church about how people can get rid of the punishment (also known as "penalties") they usually get for committing sins.
It's like saying you did something bad, like stealing candy from your friend. Normally, you would have to apologize and give the candy back to make things right with your friend. But if you also got in trouble with your parents or teacher, you might get grounded, or have to do extra chores or apologize to your friend's parents.
The same idea applies when you do something bad in the eyes of the Catholic Church. To make things right, there are different things you can do or say to make up for it, like confessing your sins to a priest and doing penance (like saying prayers, doing good deeds or donating money to charity).
But Indulgentiarum Doctrina says there's another way to make up for your sins: by doing certain things that the Catholic Church says can reduce the amount of punishment you would otherwise receive. These things might include praying certain prayers or doing good deeds.
Think of it like a "get out of jail free" card. It doesn't mean you can do whatever you want and not get in trouble, but it gives you a way to make up for your mistakes and avoid some of the penalties.
But like all things in the Catholic Church, there are rules and conditions attached. You still have to confess your sins to a priest, do your penance, and follow the other teachings of the Church. Indulgentiarum Doctrina just provides an extra way to make things right with God and the Church.