Almond flour is made from finely ground almonds, and contains no flour or other grain. Though the terms almond flour and almond meal are often used interchangeably, almond flour is usually made from blanched (skinned) almonds, while almond meal can be made from either skin-on or skinned almonds.
Almond flour is the base for classic French macarons, dacquoise, and other meringue-based sweets. It's also an important ingredient for flourless Passover desserts and for gluten- and wheat-free baking.
In some cases, you can substitute another ground nut flour, such as hazelnut, though keep in mind it will affect the flavor profile. You can also grind your own almond flour using a spice grinder, though it can be difficult to achieve the very fine texture of store-bought almond flour. You'll need to use a fine mesh sieve to repeatedly strain out the larger particles of almond.
Store almond flour in the refrigerator or freezer to keep it from going rancid.