Once signifies a state or occurrence that happens only one single time and cannot be repeated (Unique).

This is similar to the Japanese concept of Ichigo Ichie, which teaches that this present meeting happens only once in a lifetime.

Even if the same people meet again in the exact same place, it will never be that "same old time" again.

It serves as a reminder for us to appreciate the value of the present, because once a single second passes by, it becomes a "Once" (one time) that can never be turned back.