Populations tend to produce more offspring than the environment can support; and this over-production of offspring results in a struggle for survival. Animals may struggle (compete) for such things as: food, shelter, water and mating partners. Plants may compete for such things as: sunlight, nutrients, water and pollinators (e.g. bees, butterflies and humming birds).
The ‘fittest’ individuals are those with the best genes; in other words, those with the most favorable heritable variations. The ‘fittest’ individuals are the most likely to survive long enough to reproduce and pass on their genes; and the ‘weakest’ individuals are more likely to die young and not pass on their genes.
The consequence of ‘good’ genes getting passed on and ‘bad’ genes not getting passed on is that – over huge expanses of time – the best genes of a population increase while the poorer genes decline. This process of cumulative change in the heritable characteristics of a population is called ‘natural selection’ because it is the natural environment that determines which genes are the good ones.