You can highlight the text of any of your reference works, much as you can highlight text in a physical book. These highlights will remain even after you exit the program.
To add a highlight, use your mouse to select the text you want to highlight. Once the text is selected, right-click on the selection and click on the Highlighting item, which opens a submenu. Select "Highlight selection" to convert the selection to a highlight.
To remove a highlight, right-click on the highlighted text and select "Highlighting". On the submenu, select "Remove Highlight".
To change the color of an existing highlight, right-click on the highlighted text, select "Highlighting", then select "Change Highlight" on the submenu. This brings up the Change highlight color dialog. From that dialog you can select a new color.
You can select highlighted text by right-clicking on the highlighted text, selecting "Highlighting", and then click on "Select entire Highlight" on the submenu.