What does it mean to say momentum is a vector quantity and energy is a scalar quantity?

It means that in the case of momentum, the direction is relevant, in the case of energy, not.

Answer2.

It also means that science is still living in the past with respect to numbers. Scalars and vectors are quaternions. A scalar is a quaternion with an angle that is a multiple of 180 degrees. Q= cosA + vsinA = evA

Angles of 90 degrees are vectors. Other angles give quaternions, the sum of a scalar and a vector.

Energy can also be a vector like Torque = Force x Radius.