A track is a UI element that displays a value vertically or horizontally. X-Plane has three kinds of tracks: scroll bars, sliders, and progress bars. Tracks can be displayed either horizontally or vertically; tracks will choose their own layout based on the larger dimension of their dimensions (e.g. they know if they are tall or wide). Sliders may be lit or unlit (showing the user manipulating them).
ScrollBar - this is a standard scroll bar with arrows and a thumb to drag. Slider - this is a simple track with a ball in the middle that can be slid. Progress - this is a progress indicator showing how a long task is going.