Question

What does it mean to say that angular momentum is conserved?

Answer 1

The angular momentum of a system of bodies is constant if no external torques (turning forces) act on the system. The angular momentum about an axis is the linear moment multiplied by the perpendicular distance between the axis and the momentum vector.

It is the rotation equivalent of the conservation of linear momentum (=mass x velocity) where no external forces must be acting.

Answer 2

it is very important to note that this statement is only applicable when no torque acting
the time derivative on angular momentum is the net torque acting
when there is no torque acting means that change in angular momentum so initial angular momentum is same as final angular momentum