Here's why:

* Acceleration is the rate of change of velocity.

* Velocity has both speed (how fast an object is moving) and direction.

* Negative acceleration means the acceleration is in the opposite direction to the object's current velocity.

Think of it like this:

* If you are driving a car and you press the brakes, you are applying negative acceleration. The car is slowing down, even though the engine is still running.

* If you throw a ball straight up in the air, gravity causes negative acceleration. The ball slows down as it goes up, eventually stopping before falling back down.

Important Note: While negative acceleration often means slowing down, it doesn't always indicate a decrease in speed. If an object is moving in the negative direction and its acceleration is also negative, the object's speed will actually increase. However, it will still be moving in the negative direction.