I first heard it in reference to dojo practice. It is a reason to always finish a technique, even if you make a mistake or do the wrong technique. Because every interaction is unique, even between people who work out together regularly, the vectors and energy are never identical, you should always fully finish with intent.
It also means at any gathering, a family picnic for example, you should treasure the group and the individuals that are there. It is a unique gathering and it may be the last time you get to be with them. Even if you see them every week, this moment is unique as we are all changing. It encompasses the bittersweet nature of this life and the transient nature of our lives.