A scroll wheel is a button or mechanism on a computer mouse that allows users to easily and quickly move through long documents, such as web pages or word processing files. When pressed down, it acts like a regular left-click, allowing you to select text or click links. However, when rolled in either direction (usually forward for scrolling up and backward for scrolling down), the content of the document is scrolled accordingly, making navigation through long documents much easier than using the arrow keys on your keyboard.