Essentially the same thing as instant coffee (though somewhat stronger), espresso powder is not really meant to be reconstituted and drunk, but rather to be used as a flavoring element in baking.
Being concentrated, this powder will allow you to get quite a lot of flavor into foods to which you wouldn't want to add a lot of liquid. Frosting, for instance, is quite low on liquids, and adding enough actual coffee to create a coffee flavor just isn't feasible - it will screw up the consistency.
Aside from making things taste like coffee, this can also, in small amounts, be used to deepen a chocolate flavor without necessarily coming through as coffee-ness. About a teaspoon in any chocolate thing (particularly brownies) is enough to invisibly amplify the flavor. More than that, and you get into mocha territory.
I find that this powder, even when crushed as fine as I can get it, never fully dissolves into a baked good. This isn't a bad thing, as the result is tiny flecks much like vanilla bean seeds, which connote homemadeyness.
This is available in specialty food stores, baking supply shops, and of course Amazon. The cost originally put me off a bit, but it's about the same price per ounce as vanilla, and will similarly keep forever in a dark cupboard.
If a recipe calls for this, you can totally substitute Folger's Crystals or any other brand of horrid instant coffee. This is absolutely the only thing that stuff is good for (and it's surprisingly good as a flavoring element, in small amounts, surrounded by sugar and butter). Keep in mind that espresso is stronger, so you'll need to use about 1.5 times as much instant coffee in these recipes. But as always go by taste, and because adding this powder won't mess with the consistency of the final product, you can play around with it quite a bit, until you get exactly the amount of coffee flavor you like.