According to Kant, if everything is a subjective product of our minds, how can immutable truths exist?
Immanual Kant was an 18th century philosopher who provided framework and precedent for many philosophical models after himself. His view of the world was that everyone experienced it through their own subjective senses, and that we could never truly know reality because all we can do is experience what we sense, not what is truly there. However, there are things that everyone experiences the same way, like rain or wind:
We can be said to know things about the world, then, not because we somehow step outside of our minds to compare what we experience with some reality outside of it, but rather because the world we know is always already organized according to a certain fixed (innate) pattern that is the mind. Knowledge is possible because it is about how things appear to us, not about how things are in themselves. Reason provides the structure or form of what we know, the senses provide the content.
("Epistemology: Kant and Theories of Truth", philosophy.tamy.edu)
In other words, an A Priori Truth ("from the earlier") is something that is true regardless of personal experience. Since our subjective views of the world mean that we should experiences many things differently, Kant posited that there are fundamentals to the universe that we cannot understand; however, since everything is already and always "organized" according to an unknowable standard, we experience things like math and gravity the same as others, because the standards are all the same. This does not preclude different ways of viewing the world, but it allows for universal truths to exist inside Kant's model.