from evolutionary biology.
Any trait that has the effect of making an individual more fit to survive and reproduce is said to be selected for. Examples include a better ability to hide from predators or find food.
The term comes from the term natural selection. the opposite of selected for is selected against.
In a sentence: With the increased use of antibiotics in hospitals, the ability to resist antibiotics is a trait that is selected for in bacteria.
Traits that are selected for tend to proliferate - ie, we will see more it in the future.