A key is a tool to help you identify something that's unknown, or in other words, a key is a map to a place of discovery. Dichotomous means "divided into two," like a fork in a road. When you travel through a dichotomous key, it's like travelling down a road with many forks.
Each fork in the road is called a couplet (again meaning two). A couplet is just a fork in the road where the traveler has to decide which direction to go.
Imagine you're on a trip and get hungry. You're in a large city so there must be a lot of dining choices nearby, but nobody in your car is familiar with the area. You're in luck because the local city planners were thinking of you. Let's roll!
Once you've decided it's time to eat, you pull onto the "Food Road" (Think of the Food Road as "The key to restaurants in this city."). The first fork might have a sign that says "Fast Food" and "Fine Dining." You're in a hurry so you take a left here.
The next fork might have a sign that says "Burgers and Fries" and "Other types of fast food." You don't feel like a burger so you take a right.
"Asian Cooking" and "Italian Cooking." Pizza sounds good to you so you take a right (Those clouds and trees sure look familiar!).
"Pizza" and "Pasta." You take a left.
There's the Pizza Joint! See how that works? It's just a series of questions with two possible choices.
Don't worry if you change your mind after a choice. You can always turn around and go back to any of the previous forks and choose the other road.
The choices in the "KEY TO SELECTED PYRALOIDEA (LEPIDOPTERA) LARVAE INTERCEPTED AT U. S. PORTS OF ENTRY" are a little more technical, but there are images to help you along. Each fork in the road, or couplet, is numbered and the choices are the numbers of the next couplets. Keep picking the number for the next corresponding couplet until you get to the identification of your larva. You can jump back to any couplet by picking a number in the table at the bottom of the pages. Or you can simply start over at Couplet 1.