What does Darwin mean by survival of the fittest?

Answer 1

It should be noted that it wasn't Darwin who coined the phrase "survival of the fittest", and that biologists do not generally use the phrase, as it is an inaccurate description of natural selection.

Natural selection is the mechanism by which populations adapt to changing circumstances, or the mechanism that filters the genetic drift of populations to better fit existing circumstances.

Natural selection is the term used to describe the differential reproduction of variant alleles. Alleles that affect the reproductive capabilities of the organism that bears them positively will increase their frequency in the population gene pool. Alleles that affect their own reproduction negatively will decrease their frequency.

Answer 2

Living beings srtuggle to survive and pass on their genes to the next generation, which will do the same.