When creating an autoresponder or editing a thank you page on a donation form you may want to have the amount donated inserted in a specific place.
[[S120:dc:giftAmount]] is the S-Tag that will bring this value in when the value is generated in the donation session. If you use the WYSIWYG editor, the value can be found to insert under personalization (the little human icon), Donation > Gift Amount.
The definitive list of what S120 properties are available can only be found by entering [[S120:help]] wherever you want to use it. Then take whatever action is necessary to get to the page or autoresponder that you have entered the S120:help tag on. That will give the list of what values are actually available where you are attempting to use the tag.