Question

what is angular acceleration?

Answer 1

Suppose something is spinning with an angular velocity of 2rad/s.
It speeds up, and after 5s it is spinning at 17rad/s.

Angular acceleration = (final angular velocity - initial angular velocity) / time
= (17-2)/5
= 3 rad/s²

So it is calculated in a similar way to 'ordinary' acceleration.

For uniform circular motion, angular velocity is constant, so angular acceleration = 0.
(Just like uniform linear motion: velocity is constant means acceleration = 0.)

Answer 2

my guess would have been that its acceleration that produces a change in direction. But then saying angular acceleration might seem a bit redundant in that case.