When we say that a wave is a disturbance that travels through a medium, it means that the wave is the propagation of energy or information through a physical medium by causing a disturbance in that medium. The disturbance itself is created by an external source or an initial perturbation.
Let's take the example of a water wave. When you drop a stone into a still pond, it creates a disturbance in the water. This disturbance consists of ripples or waves that spread out from the point of impact. In this case, the stone is the external source that creates the initial disturbance.
Similarly, in other types of waves, such as sound waves or seismic waves, an external source or event is responsible for creating the initial disturbance. For instance, in the case of sound waves, the disturbance is caused by a vibrating object like a loudspeaker or vocal cords, which produces compressions and rarefactions in the surrounding air molecules.
Once the disturbance is created, it propagates through the medium by transferring energy from one particle to another. Each individual particle in the medium oscillates or vibrates around its equilibrium position, passing on the energy to neighboring particles. This transfer of energy allows the wave to travel through the medium without the actual displacement of the particles themselves (except for certain types of waves like water waves).
So, in summary, a wave is a disturbance that travels through a medium, and the disturbance itself is created by an external source or an initial perturbation. The wave then carries the energy or information associated with the disturbance as it propagates through the medium.