1) QFT is an expression of agreement and support, where the user stands behind you and one of your statements. This commonly occurs in controversial topics where opinions are very heated, and people will choose sides in an argument. If someone "quotes you for truth", they are paying you a compliment, and siding with you in the discussion.
2) QFT can also be used to preserve an original forum post, so that the original author cannot edit after the fact. A user who copy-pastes the original forum content will sometimes put the letters "QFT" at the top of the copy-paste. It is a type of forensic stamp, commonly used during very heated discussions on a controversial topic. The original author can no longer change their original text, because it is now preserved through someone else's post. The original author is prevented from denying what they originally wrote because the QFT public copy can refute any denial.