What is neutral selection?

Neutral selection is the changes in a gene pool of a species that are a result of random neutral occurrences that do not give any advantage to that species. Neutral selection does not depend upon adaptation, fitness, or natural selection.
If you are referring to the 'Nearly Neutral' theory of natural selection, then read on. If this is a typo for 'Natural Selection', skim down to the next paragraph. The 'Nearly Neutral' theory of evolution states that for a change to come about in the population as a whole, the new characteristic must be better than or equally good as the old characteristic for the change to occur. If, for example, a population of dog has brown eyes, and a new, mutant, blue-eyed dog arrives, then this is not going to be a worse characteristic than brown eyes. This means that the concentration of the blue-eyes in the population can go on a 'random walk' and may become the norm.
Natural selection is the main idea behind evolution. Basically, it is based on several observations:
1 - Some members of the species are different from other members of the species
2 - Parents look like their children
3 - Some characteristics will help survival
With these in mind, it is clear that those who are better equipped will tend to survive more and pass on their characteristics to their children. That's Natural Selection.