A "hammer-on" is a guitar technique used to create a smooth transition between notes. It involves playing a note by plucking the string and then using a finger to press down on a higher note without picking the string again. This technique allows for faster playing and adds a fluid sound to melodies. Hammer-ons are commonly used in various music genres, including rock, blues, and jazz. They can enhance solos and riffs, making them more expressive. Guitarists often practice hammer-ons to improve their finger strength and dexterity, which are essential for advanced playing techniques.