Here's a breakdown:

* Relative Motion: The key is that the objects are moving *in relation to each other*. It doesn't matter if they are both moving or if one is stationary.

* Frames of Reference: Each moving object can be considered a separate "frame of reference." Velocity addition helps us understand how the velocities of objects appear in different frames of reference.

Examples of when velocity addition is used:

* A car traveling on a moving train: The car's velocity relative to the ground is the sum of its velocity relative to the train and the train's velocity relative to the ground.

* A boat crossing a river: The boat's velocity relative to the shore is the vector sum of its velocity relative to the water and the water's velocity relative to the shore.

* A plane flying in a wind: The plane's velocity relative to the ground is the vector sum of its velocity relative to the air and the wind's velocity relative to the ground.

Important Note: Velocity addition typically involves vectors, as velocity has both magnitude (speed) and direction. Therefore, we use vector addition to combine velocities accurately.