When editing pages this option enables to use the HTML editor (called the editor from this point) instead of a simple textarea box. 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 check-box next to Use HTML editor.
If the editor is required, first click on the Use HTML editor box which should then contain a tick. The page is then re-displayed to activate the use of the editor.
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 HTML 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 HTML 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.