An acquired trait is a physical characteristic of an organism that is not passed down to offspring genetically. It is not coded in the organism's DNA and is a product of the environment's influence on the organism.
These traits can strongly influence the survival of an organism. Acquired traits can cause an organism with a genetic disadvantage to out-compete another. An example of an acquired trait is a bodybuilder's large muscles. They were acquired by lots of exercise and not genetics, so the offspring of the body builder are no more likely to have large muscles than other offspring. Since acquired traits cannot be passed on to offspring, they do not increase the fitness of a population, and natural selection does not influence them.