This means to press the primary mouse button (almost always the left one) down, then release it.
This means to rapidly press the primary mouse button down, release it, press it down again, and release it again.
This means to press the right mouse button down (unless you are using a left-handed mouse), then release it.
This means to press the primary mouse button (almost always the left one) down on something, hold it down while you move the mouse somewhere else, then release the button.
The definition of highlight is to mark an item or segment of text. How you do it depends on what exactly you are highlighting. If you wish to highlight something in a list or selection box, then click on it. If you wish to highlight all or part of some text in an edit box or text window, you can drag the mouse from one end of the desired text to the other. You can also use the keyboard to highlight text: place the keyboard cursor at the beginning of the text you wish to highlight, then hold down the Shift key while you move the keyboard cursor to the end of the text you wish to highlight.
Note in the above example that I have highlighted the words "Then was", and " the time" remains unhighlighted. If you look carefully, you can also see that the keyboard cursor is between the "s" and the mouse cursor, which is presently in the shape of an I-beam.