In evolutionary optimization, an "individual," also termed a "genotype" or "chromosome," is a candidate solution to a problem. Think of it like a recipe where each ingredient is a parameter of the solution. These parameters are the "genes" of the individual. Together, they define how the solution behaves or performs. Like in natural evolution, these individuals can be altered (mutated) or combined (through crossover) to create new solutions, in the quest to find the best or most effective answer.
Population is a set of individuals.