What does it mean to be selected for or selected against in an evolutionary sense?

In an evolutionary context, being "selected for" means that certain traits or characteristics increase an organism's chances of survival and reproduction, leading to those traits becoming more common in a population over time. Conversely, being "selected against" refers to traits that decrease an organism's fitness, making it less likely to survive and reproduce, which can lead to those traits becoming rarer or disappearing from the population. This process is central to natural selection, driving the evolution of species.