When editing pages this option enables the use the HTML editor (called the editor from this point) instead of simple textarea boxes. The editor allows the simple formatting of text and the easy addition of "advanced" features such as links.
The editor is switched on by clicking on the box next to Use Editor. The editor is associated with the items on an individual basis. For example, switching the editor on for, say, the Response 1 item just sets the editor on for that item, it does not set it on for all the Response boxes.
If the editor is required for some of the items of a page, first click on the Use Editor boxes (They should then contain ticks.) The page is then re-displayed to activate the use of the editor. This is done by clicking on the Redisplay Page button at the foot of the page.
Note that care should be taken when using the editor. In most cases it should not cause any problems. However, it is not recommended for Answers of Short Answer and Numeric type questions. The editor can add extra "invisible" HTML tags which stop comparisons working as expected. Further, the Descriptions in Branch Tables are used in Buttons and again if they contain any HTML tags this can cause problems. There are no problems with Answers which are used only for display, such as those in Multichoice type questions. Again use of the editor for Response items should not cause any problems.
If a problem is suspected with any of the text items it is quite safe to switch the editor off (by removing the tick in the Use editor box). When the page is redisplayed, the "raw" text can then be seen in the textarea box. The text can be edited as required. In fact, switching the editor on and off repeatedly for a particular item should not change the text.
The "state" of the Use editor option is saved with each item. Thus if a page is re-edited the items which were using the editor will again appear within the editor.
Finally, the editor is quite "expensive" in terms of resources within the browser. If items do not require the use of the editor it is quicker and less prone to resource problems to just use the standard textarea boxes.