A change was made to the kernel that broke some applications - this is called a regression (a bug introduced in an update). The developer who made the change that caused the bug in the downstream application tried to argue that it was the application's fault, not the kernel.
Linus then calmly and politely reminded him that the official policy of kernel development is that they never introduce changes that break applications.
You only say "calmly and politely" because Linus is a known pottymouth and frequently "tears people a new one". An email like this might get you demoted or fired in some workplaces.
regression = "it worked before, it doesn't work anymore"