Coaxial means it shares an axis. You can imagine these as circles that have either the same centre in 2D or whose centres belong to a common line in 3D (where the circles are probably assumed to be parallel if so, check that).
Edit: As Lord Farin has said, it could also very well mean that the centres all lie on the same line even in $2D$. Go through the paper a bit more and see if you can figure it out.