What does ... mean in numpy code?
Yes, you're right.  It fills in as many : as required.  The only difference occurs when you use multiple ellipses.  In that case, the first ellipsis acts in the same way, but each remaining one is converted to a single :.
And what is it called? I don't know how to search for it; I tried calling it ellipsis with the Google.  I don't mean in interactive output when dots are used to indicate that the full array is not being shown, but as in the code I'm looking at, 
xTensor0[...] = xVTensor[..., 0]
From my experimentation, it appears to function the similarly to : in indexing, but stands in for multiple :'s, making x[:,:,1] equivalent to x[...,1].