Strength
The physical ability to overcome an external resistance by muscle force production. e.g. The heavier the weight you can pick up from the floor, the stronger you are. Strength training refines you as a person because character is built by taking on challenges that are hard.
Nutrition
You are what you eat. The right nutrition is just as important as the right training. No matter whether you want to loose body fat or gain muscle mass. I lowered my bodyweight from 102 kg to 82 kg through healthy nutrition and training. I know how to gain muscle and loose fat from my own personal experience.
Training
Training is a physical activity with a long term goal in mind. Each Workout is precisely designed to reach that goal. Training is not to be confused with exercise which is physical activity done for the effect it produces today.
https://startingstrength.com/article/from-the-archives-training-vs-exercise
Progressive Overload
"Stress-Recovery-Adaptation" is the way your body gets stronger. You lift a weight you can hardly lift and in response your body adapts by getting stronger, so next time you can lift a sligthly heavier weight.
Further imformation: https://startingstrength.com/article/the-mathematical-nature-of-strength-training
Recovery
You get stronger by recovering from training, not by the training itself. For this reason training 2-3 times per week is the sweet spot for progress.