This is the sign that warns of a snake to steer away from. The triangular head marks a snake with fangs. Sounds simple, but you do need to see them clearly.
This is a Cottonmouth (Water Moccasin) and is a poisonous viper. Snakes look a lot alike, and most are really important to this environment. I just look, and move on.
Just for fun, above is where they like to live. A swamp. Remember that part about seeing them clearly.