Do you mean Charles Darwin, the evolutionist? I am not sure that he was a "sir", although one of his grandsons was and some of his other relatives.
While I am not an expert on evolution, my understanding of his use of the term "survival of the fittest" is that on his voyage on "The Beagle", he saw many individuals in many species, including human kind, all of which were different from one another. Think about people - you do not see two people who are absolutely identical (even identical twins often have some points of difference, although some of these may not be visible). His thoughts on this were that some individuals had attributes or characteristics that made it easier for them to survive and produce young or to produce MORE offspring than other members of the same species. So, for example, if a bird had a slightly longer, thinner beak than its nest mates, it might be able to probe deeper into bark for insects and so, would get more food, would be stronger and maybe more likely to survive than its brothers and sisters. Because it was stronger (fitter) it might be able to have stronger offspring (more food) or be able to produce more offspring. Provided that thinner, longer beak was passed onto its offspring, they also would be able to get more food and so would be "fitter" than other birds of the same species. In this case, Darwin's theory suggests that the characteristic of "longer, thinner beak" would become widespread in this species - the fittest surviving longer and better and so able to produce more and stronger offspring with their same characteristic.
Of course, if the longer, thinner beak means that it gets damaged more easily, then the bird is less fit than others. If it damages its beak, it may not be able to get food. It will die and not produce offspring.
Thus - the fittest creature (or plant or other living entity) is the one that has characteristics that make it able to produce more offspring that survive.
