Fitness is good physical condition. If you're a fitness nut and you're constantly working out, everybody else at the gym might be eyeing your sculpted muscles with envy and marveling at your stamina and strength.
Fitness can also refer to your being fit, or qualified, for something. If you show up in ragged jeans and a dirty t-shirt to interview for a job in the clean room of a microchip factory, your interviewer might question your fitness for the position. For a ship, fitness is a measure of whether it's seaworthy. In biology, an organism's fitness for a particular environment is its ability to survive and reproduce in those conditions.