Papal indulgences must be preached with caution, lest people erroneously think that they are preferable to other good works of love.
This is basically saying that if people can casually get papal indulgences they will use them and do bad things instead of doing more productive methods to get forgiveness. It is also saying that people can just do bad things as long as they can pay for an indulgence which is bad. He understands repentance as something that must be genuine, and thinks selling indulgences makes the repentance not legitimate