Acceleration is defined as the rate of change of velocity
Since velocity is a vector, it has both speed and direction.
So, if the object is travelling in a straight line at a constant speed, then it will not be accelerating. Both Magnitude and direction are constant.
However, if the object is travelling in a circle at a constant speed, then it'll be accelerating. An Object going in a circle is constantly changing it's direction, which means velocity is changing, so there is an acceleration caused by a net force.
Hope this Helps :)
