Saying that a wave has a frequency of 25 hertz means that it completes 25 cycles per second.

Here's a breakdown:

* Frequency is a measure of how often something repeats. In the case of waves, it refers to how many complete cycles (from peak to peak or trough to trough) the wave completes in one second.

* Hertz (Hz) is the unit of measurement for frequency. One hertz equals one cycle per second.

So, a wave with a frequency of 25 Hz oscillates (goes up and down) 25 times every second.

Think of it like this:

Imagine a pendulum swinging back and forth. If the pendulum swings 25 times in one second, it has a frequency of 25 Hz.

Examples:

* Sound waves: The frequency of sound waves determines the pitch we hear. A wave with a frequency of 25 Hz would be a very low-pitched sound.

* Light waves: The frequency of light waves determines the color we see. Higher frequencies correspond to colors like blue and violet, while lower frequencies correspond to colors like red and orange.