Fitness
Your fitness is how good you are at passing your genes on to future generations. It's mostly based on how well you are able to survive and how many offspring you are able to have, but sometimes more subtle traits come into play. For example, an individual who helps her siblings to survive and produce offspring may be indirectly increasing her own fitness, since siblings share half of one's genes. In everyday speech, the word "fitness" means the condition of being healthy and athletic. This is similar, but not identical, to the meaning in evolutionary biology (healthy, athletic individuals are often good at passing on their genes, but sometimes the scrawnier, cleverer ones are sneaking behind the big guys' backs and getting all of the mates). Genes don't determine every aspect of fitness (for example, an individual may by chance encounter a deadly virus and die, while another genetically identical individual luckily avoids the virus). However, when alleles differ in their effects on fitness, the allele that promotes higher fitness will tend to become more common; this is natural selection at work.