What does frequency mean in science

There's nothing mysterious about it at all. "Frequency" simply means

"often-ness" ... how often or how frequently something happens.

- - The frequency of traditional meals is 3 per day.

- - The frequency of an equinox is 2 per year.

- - The frequency of my sleeping really late is 1 per week.

- - The frequency of my intense desire to sleep late is 30 per month.

etc.

- - The standard unit of frequency in the SI system is "per second".

The special name for that unit is "Hertz". (Hz)

A rate at which a cycle or phenomenon is repeated.