The important part is the last two words, proper time, which is the time measured by a clock travelling along some given world line [1]. In this case, the world

The important part is the last two words, proper time, which is the time measured by a clock travelling along some given world line [1]. In this case, the world line of the hypothetical interstellar craft. [1] https://en.wikipedia.org/wiki/Proper_time https://en.wikipedia.org/wiki/Proper_time