Good question, I've never heard it before. Hopefully somone with more understanding than I will answer it. To expand your question, Earth isn't exactly standing still is it? And if the universe is expanding at a high rate of speed, then the space traveller could actually be traveling through space at a slower rate than the person on earth? eh?
'Very interesting from a biological POV. Though it doesn't seem to change the physics.
A "frame" in SR consists of a network of local observers at rest with respect to one another, who synchronize their clocks via light signals in straightforward fashion.
If one such frame, or network, is spread out along a straight line, with constant separation between neighbors, and another is spread out along the same line, in the same way, but moving at a steady speed wrt the first frame, the observers in both frames will observe the clocks of the other frame to be running slow, compared to their own physically identical clocks.
Both sets of observers will observe the other frame's clocks not to be synchronized, and both sets of observers will observe the other frame's observers to be closer together than they themselves are.
The whole thing works out mathematically in an exquisite symphony of eighth grade algebra. Such is genius.
"To expand your question, Earth isn't exactly standing still is it? And if the universe is expanding at a high rate of speed, then the space traveller could actually be traveling through space at a slower rate than the person on earth? eh?"
It has to do with accelerated frames of reference. The object that undergoes acceleration to a higher velocity, relative to the observer, will experience less time passing than the non-accelerated observer. If the two twins were to undergo the same accelerations, say in opposite directions, and then come back together, they would both have aged the same amount, which would be less than the non-accelerated observer aged.
Note that gravitational fields also cause an acceleration, and they also cause relativistic time effects. IIRC, the greater the gravitational field you're in, the slower time runs. So satellites in orbit around the earth experience time at a different rate than we do on the surface, not only because of their movement, but also because the gravitational field they experience is not as strong.
HTH.