Invisible tags are a useful way you can influence search results. Like normal tags they will contribute to search ranking of an FAQ, but they won't be shown on the front-end to your users. If you therefore want a particular phrase to return a particular FAQ but your content doesn't feature that phrase or is being out ranked by another FAQ invisible tags can help.
You can make current tags invisible or set up new invisible tags in MyMalcolm via FAQs / Tags:
You then apply tags as usual via the configure area of the individual FAQ: