What does the * mean when creating a giveaway.

Have a browse.

To answer your question: It means that it is a game from an indie bundle. Examples are from Humble Bundle, Indie Royale, Indie Gala, etc. Generally when they have an asterisk that means if you give ONLY games from an indie bundle then your contributor value won't get past $30. Please note that games that are tradable/gifts/keys that aren't from the bundle that are bought from Steam/Gamersgate/GMG will now be considered "bundled" games.

Also you can increase your bundle value by giving away games that aren't only from bundles, I'm not quite sure what the equation was for that but you should be able to locate it with the search feature on the forums.