You expect that natural selection will favor the genotypes that make an organism more fit and select against those that make it less fit. For example, plants with better roots get more water and make more seeds, and natural selection drives the population to have better roots from one generation to the next. But maybe there are different ways to make a good root (or anything else in the biological world).
The adaptive landscape is a way of thinking about the fact that several peaks of high fitness may exist, each peak corresponding to a different genotype. Natural selection will drive a population to the top of whatever peak it's already on, whether or not that peak is the highest one in the adaptive landscape.
What natural selection won't do is move the population to an even higher peak if that move means crossing a valley of lower fitness. Why? Because natural selection won't backtrack by favoring individuals with lower reproductive success. Individuals with lower fitness arise all the time (remember that most mutations are bad), but they don't increase in frequency as a result of selection.
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