Scroll wheel is a wheel, made of rubber or hard plastic, given on every mice on the top surface mostly in the middle of both mouse buttons. The primary function of the mouse wheel is to move upward or forward by scrolling the wheel up, and downward or backward by scrolling down.

https://cpstest.us/scroll-test/

What is Horizontal Scrolling & Vertical Scrolling?
Basically, there are two types of mouse wheel scrolling which are being used;

Vertical Scrolling
Verticle Scroling is the primary method which is available in every mouse. This method enables to view the content up or down by scrolling the wheel upward and vice versa to view the content down.

Horizontal Scrolling
Horizontal scrolling is something rare and not available in ordinay mices. In order to test horizontal wheel scrolling we need a special gaming mouse. A mice with horizontal scrolling enables the user to scroll the content view in both directions left or right.