Natural selection has a single meaning whether in the context of viruses or anything else. I think you should read the whole explanation, but it boils down to this: 1) organisms randomly mutate; 2) useful mutations are selected for better fitness and kept in the population.
I think it is great that you are reading so many articles. Still, I don't think this forum is meant for answering so many questions of the type
what is the meaning ...?, especially when they are easy to answer by Googling and when they are unrelated to bioinformatics.