On a speed time graph a horizontal line shows the change in speed is?

A horizontal line parallel to speed axis indicates that the body is moving at a constant speed i.e. it's speed doesn't change with passage of time. The body may have zero or non zero acceleration, but it will always have non zero velocity.

But, that doesn't mean that body's velocity is constant and it is not accelerated. For example in uniform circular motion the speed of body remains constant but velocity changes due to change in the direction of motion. Also there is acceleration towards center of circle called centripetal acceleration which changes the direction of velocity.

For this acceleration a force should act towards the center called centripetal force.

If speed is constant then the force, if present, must act in direction perpendicular to the immediate direction of motion i.e. perpendicular to the tangent drawn at any point on the path of motion.