Why is a car rounding a curve accelerating, even if it is moving at a constant speed?

Speed is just a measure of how fast something is going, while velocity is more precise and describes both how fast something is moving AND in what direction.

Therefore if an object is changing direction (i. e. going round a curve), even if it is at a constant speed, its velocity is still changing (i. e. the direction it's moving in changes). And any change in velocity is caused by an acceleration, therefore the car must be accelerating to go round a curve.