What did charles darwin mean when he said some organisms where more fit than others

Darwin's idea of the survival of the fittest means that certain organisms are more likely to survive with they traits in particular environments. This is because some organisms are more fit to live in a given environment whether it be due to temperature, weather, or predators.

One example is that dark moths are more fit for survival than light moths in many areas because they are able to camouflage against dark tree bark.

Hope this helps.