A hardware component providing a surface that allows software to display any manner of buttons and other interactive elements on a visual display, that you can touch directly, and the software will respond to that touch. Touch screens are a staple of modern smartphones.
Touch screens may include a transparent touch-sensing layer just above the display layer, or the touch layer may be integrated with the display, or even below it.
The most common type is capacitive, which is designed to sense to the electrical properties of a human fingertip and does not require any pressure/force applied. Most capacitive displays also have a feature called multi-touch, where they can respond in unique ways to more than one simultaneous finger touch.
See: Capacitive Touch
Some touch screens also respond to a stylus, which may require an additional sensor layer or component, depending on the type of stylus. A stylus may be used for drawing or taking notes, for example.
An alternative touch screen technology that has fallen out of favor is resistive. The earliest smartphones used this type, before capacitive eventually became standard. These touch screens required a small amount of pressure/force applied, but would respond to any object, not just fingers.
See: Resistive Touch Screen