"Clicking and dragging" is a way to move certain objects on the screen and to highlight text or images. To move an object, place the cursor over it, press and hold down the left mouse button, then move the mouse while still holding down the left mouse button. When you have "dragged" the object to the location you want, let go of the mouse button.
Try clicking and dragging the box on this page.
Now position the cursor over this paragraph of text. Hold down the left mouse button, and move it over the text. Notice that you have highlighted what you moved the mouse over.
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