Not all of the individuals in a species are exactly the same. There are variations, differences, among members of a species. If you look at the spots on several different ladybugs, or the stripes on zebras, you will notice that they don't all have the same number or arrangement of spots or of stripes. In addition to these easily visible variations, there are differences in skill and behavior, such as differences in how fast the zebras can run. If organisms were all the same, none would be better suited than any other, and selection could not occur.
Some variations allow members of a species to survive and reproduce better than others.
If an organism has a trait that helps it survive or reproduce, it is more likely to survive and be able to reproduce. A faster cheetah is more likely to catch a gazelle and survive, and a faster gazelle is more likely to escape the cheetah and survive. A showier flower is more likely to be noticed by a bee, which enables it to reproduce. A thornier cactus is more likely than other cactuses to be left alone by animals, rather than be eaten and die. Organisms that survive and reproduce pass their traits to their offspring, and the helpful traits gradually appear in more and more of the population.