The sex ratio gives the ratio of males to females in a population. In most species, the sex ratio is about 50:50. This is an equilibrium point: if a population ever comes to deviate from it, natural selection will drive it back.
At first sight, the 50:50 sex ratio might seem inefficient. Most species do not have parental care and are not monogamous; a male can fertilize several females.
Why are there so many males?
It is one of the most successfully understood adaptations. The main idea is due to Fisher: if there are fewer males than females, then males have a higher chance of mating. There is an advantage to being a male, and an advantage to a female who produces extra sons: sons have a higher reproductive success than daughters, and natural selection will favor mutant females that produce more males.
The same argument applies when there are fewer females, so the sex ratio stabilizes. Any population that deviates from the 50:50 sex ratio will be shifted back to it by natural selection.