It is the energy transferred upon applying a force of one pound-force (lbf) through a linear displacement of one foot.
In the United States this is often used to specify, for example, the tightness of a fastener (such as screws and nuts) or the output of an engine.
Although they are dimensionally equivalent, energy (a scalar) and torque (a Euclidean vector) are distinct physical quantities.
Both energy and torque can be expressed as a product of a force vector with a displacement vector (hence pounds and feet); energy is the scalar product of the two, and torque is the vector product.
In small arms ballistics and particularly in the United States, the foot-pound is often used to specify the muzzle energy of a bullet.