Regeneration is simply the renewal of cells in the body even regular humans perform regeneration. However regeneration defferes from organism to organism. For example one can't compare a jellyfish's regeneration to a lizard, it is obvious the regeneration of the lizard is no where near that of the jellyfish.
It isn't correct, being resistant to physical damage =/= immortality. Tsuande can be killed by drowning, strangulation, soul removal, chakra absorbtion etc. Admit it she was bragging simple as that, it is something almost every character does I am not faulting her on that but character statements have been proven wrong before and what characters say shouldn't be taken as 100% true.