What is the poem about coral snakes?

Assuming you're talking about the rhyme to remember the colour bands - to check which snake you're looking at...

Red touches yellow, can kill a fellow

Red touches black, venom lack

The colour bands on a Coral snake are in the order red-yellow-black-yellow.

Those of the milk snake are red-black-yellow-black- The order of these bands is repeated down the snakes body.