Text Highlight

SS of drop-down from Home tab, Font group

Whenever you start Word no highlighting will be applied and the Stop Highlighting button will be disabled

Depending on what is selected when you choose your colour will determine if you highlight multiple pieces of text or just highlight a single piece of text

Select Text First

If you select your text before pressing the button then it only highlights that piece of text

SS example

Nothing Selected

If nothing is selected and you press the button then the cursor will change to a small paint brush and you can highlight multiple pieces of text.
You can keep highlighting text until you either press ESC or you select Stop Highlighting
SS example
Stop Highlighting - Alternatively you can press the ESC key
This ESC functionality is similar to the ESC when you are using the Format Painter.

Important

The last colour that was chosen is saved and is chosen by default the next time you open Word.