Favorable traits are traits that promote an organism's success in a particular environment. Organisms with favorable traits are more likely to reproduce and pass on their traits than organisms without favorable traits. In this way, the external forces of nature determine, or "select" which traits (or even which types of organisms) will continue to exist in a population.
Favorable traits may include physical traits, such as the body of a sea lion storing extra fat, or they may be instinctive behavioral traits, such as a bird building a nest to protect its young.
If an organism dies before reproducing, then its unique traits will be eliminated from the population. On the other hand, if an organism has favorable traits that allow it to survive and produce many offspring, then its traits will be more numerous within the population.