This phrase is a mnemonic device to remember the major difference between water hemlock and Osha or Angelica. All three are plants in the carrot family, and they all have similiar shapes, with hundreds of tiny white flowers projecting from their stalks (much like their relative, Queen Anne's lace). The difference, being, of course, that water hemlock is deadly posionous while Angelica and Osha are yummy and nutritious.
And one of the few ways to tell the difference is through the patterns of venation on the leaves. Water hemlock has veins that go to the bottom of the minute serrations in the leaves, while Angelica and Osha have veins that go to the tips of the serrations. Thus the rhyme.
All this being said, the best advice (unless you are out in the wilderness, about to starve) is to forego any of these plants altogether.