When talking about fitness, people have a certain image in their head on what constitutes a fit person. To be truly fit, a person should be strong in all the five pillars, or areas, of their fitness.
Cardiovascular endurance is how efficiently your heart and lungs work together with the rest of the body (i.e. muscles).
What can improve this are activities like running or sprinting, swimming, or cycling.
Muscular strength refers to how much force a muscle can exert in a low amount of reps.
To improve muscular strength, you should do resistance exercises such as the bench press, barbell squat, etc. with a low amount of reps and a lot of weight.
Muscular endurance refers to how long a muscle can be in continuous effort without burning out or fatiguing.
To improve muscular endurance, I recommend exercises like push-ups, pull-ups, lunges, and squats.
Flexibility is referring to how far a joint can perform a movement in full range of motion.
Exercises to improve this can be yoga and stretching.
Body composition is how much fat you have in your body in relation to how much muscle, organs, and bones. It is possible to test it with pinch tests and underwater weighing, which is quite expensive.