A skeuomorph is a decorative object that retains the design of the original object when the design elements of the original are no longer necessary. In physical design, the inclusion of molded rivets, screw heads, or other non-functional elements that were present on the original objects that were crafted from metal or wood, but are now crafted from plastic, are examples of skeuomorphs.
Skeuomorphs are even more obvious in digital interfaces, however, as there is typically little reason to clone the interface of an analog device for a digital interface beyond tradition or design aesthetics. Among the most obvious examples of this are found among the many music applications that include a digital representation of the analog interface found on the original mixing equipment they’re imitating—the interface above is from the GEARcompressor audio plugin from Redstair, for example.
Slides and knobs make sense on a physical device, but aren’t necessary on a digital device that lacks actual hardware pieces and can provide a higher degree of precision in selection than an analog knob, slider, or lever. They do, however, represent a layout and interface that people who have previously used the analog versions of the devices are familiar with, so the design element persists.