Trail mix started off as a nutritious, healthy, carbohydrate-rich, high-energy snack of dried fruit and nuts that could be munched by hikers on the go.
It's since morphed into something that's more upmarket and far less healthy - but probably much tastier; you can find commercial trail mixtures that contain chunks of good-quality dark chocolate, pieces of cashew, toasted coconut and dried sour cherry. And why not? If you're going to eat something - even if it's just for the energy - it might as well be as delicious as possible.
Trail mix is so easy to make it puzzles me why anybody would bother to buy it - but they do. You'll find it in grocery stores, in the section devoted to energy bars (which contain more or less the same ingredients, but in compressed form).
To make your own, mix together whichever nuts, seeds, dried fruit and toasted grains you like, throw the lot into a zip-lock bag (or a lock-in plastic container, if you're environmentally friendly) and put that into your backpack, to take on a hike (don't forget to take plenty of water, too). The snack shouldn't be limited to hikers, of course - it's good for anyone who needs an energy boost at any time.
But if you spend your day sitting behind a computer and the longest walk you take is to the coffee machine, remember trail mix is considered high energy because it's high in calories.