Here's a breakdown:

* Hertz (Hz): The standard unit of frequency, representing one cycle per second.

* Nano (n): A prefix in the metric system meaning one billionth (1/1,000,000,000).

Therefore, a nanohertz is an extremely low frequency, equivalent to one cycle per billion seconds.

Here's why it's important:

* Astronomy: Nanohertz frequencies are used in the study of gravitational waves, which are incredibly faint ripples in spacetime. These waves are generated by massive cosmic events like the merging of black holes and neutron stars.

* Other applications: Nanohertz frequencies are also used in fields like:

* Earth sciences: For studying the Earth's rotation and internal structure.

* Materials science: For understanding the behavior of materials at very low frequencies.

In summary: A nanohertz is a minuscule unit of frequency, often used in the study of phenomena occurring over extremely long periods of time.