Speed tells you how fast an object moves. Velocity tells you how fast and in which direction. Speed is a scalar quantity because it has size only, while velocity is a vector quantity because it has size and direction.
An object moving in a circle at constant speed, such as the Moon orbiting Earth or a satellite in a circular orbit, is always changing direction. At every moment its velocity points along the tangent to the circle, and this direction keeps turning. Velocity at opposite sides of the orbit points in opposite directions.
So the speed is constant but the velocity is changing. A change in velocity is an acceleration, so the object is accelerating even though it is not speeding up or slowing down. At this level you only need to explain this idea in words, with no calculations.
