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 of the fretting hand to press down on a higher fret 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. Practicing Hammer-Ons helps improve finger strength and coordination, essential skills for any guitarist.