To scroll something is to move it up or down the screen, so you can see what is above or below the current position, for example at a website. A scroll-bar is a bar on the right-hand side (usually) of a window which allows you to move the- text etc up and down the screen with a mouse. If the window is too wide to fit the screen there will be a scroll-bar at the bottom, so you can scroll the text left and right too.