Fitness is a measure of how well organisms survive and reproduce, with emphasis on “reproduce.” Officially, fitness is defined as the number of offspring that organisms with a particular genotype or phenotype leave behind, on average, as compared to others in the population.
Survival is one important component of fitness. In order to leave any offspring at all in the next generation, an organisms has to reach reproductive age. For instance, in the example above, brown rabbits had higher fitness than white rabbits, because a larger fraction of brown rabbits than white rabbits survived to reproduce. Living for a longer period of time may also allow an organisms to reproduce more separate times )e.g., with more mates or in multiple years).
However, survival is not the only part of the fitness equation. Fitness also depends on the ability to attract a mate and the number of offspring produced per mating. An organisms that survived for many years, but never successfully attracted a mate or had offspring, would have very (zero) low fitness.