Strength training is a popular term for exercises that build muscle by harnessing resistance - that is, an opposing force that muscles must strain against.
Strength training is sometimes called resistance training, progressive resistance training OR weight training.
Resistance can be supplied by your bodyweight OR by using free weights such as dumbbells and weighted cuffs OR using bands OR specialized machines you'd find at the gym.
No matter what kind of resistance you prefer, putting more tension than the usual amount of load on your muscles will make them stronger.
These exercises can strengthen your bones as well because your muscles are attached to underlying bones and this force, stimulates the bones and they get stronger.
Strength training is NOT just for 20-somethings in search of buff bodies or looking "bulky" or "big." While it can certainly contribute to reshaping a person's silhouette in a more toned, lean and sculpted way - it can more also help boost strength that everyone needs for daily tasks.
It can get to a point where just about any activity becomes easier with stronger muscles.