You're referring to Linus Torvald's first rule of kernel development. This note explains it: https://lkml.org/lkml/2012/12/23/75. I.e., when maintaining the kernel, do not do something which breaks user programs/applications. In other words, when making kernel changes, it is very bad to cause problems in the user's application "space". That doesn't literally mean memory. That means anything that impacts the user applications in a way that negatively affects its behavior (causes the program to malfunction). The note I cite also indicates at least one example.
