Inherited traits are traits that you are born with; they are in one's genes. Natural selection is the concept of "Survival of the Fittest" where only those who have the most beneficial of genetic traits survive to pass on their genetic code onto the next generation.
Therefore, only inherited traits, not acquired ones involved in the process of natural selection because the offspring of an organism with a desirable inherited trait is more likely to survive than the offspring of an organism with a desirable acquired trait as the offspring of an organism with a desirable acquired trait will not have its parent's desirable trait.