A mouse is used to click on items. As you move your mouse over an item, usually something changes to indicate that it can be clicked, such as to the cursor or the background colour.
When you move your mouse over an app, file, or folder, the background of the item, and its border, will change colour. Try it on the items on the right.
Google
Chrome
Microsoft
Word
My
Documents
What about when you use a web link or button? What happens to the mouse cursor when you move it over each of the links on the right?