Sorry, it's a common "term of art" in the food sciences.

Basically, it is blindly tasting three samples, and trying to tell which one of the three is different. (The other two are the same.) So, if you are testing a new supplier, you can give the subjects two samples from the old supplier and one from the new supplier, and see if they can pick out the odd one with any kind of statistical significance.

The key factor is that it is not attempting to measure "which tastes better", but rather, "which tastes noticeably different".