Complex sentence: Rotational speed tells how many complete rotations (cycles) there are per time unit, and it is measured in cycles per second ("1/s " or"Hertz") in the SI System.
Split into simpler sentences: Rotational speed tells how many complete rotations (i.e. "revolutions" or "cycles") there are per time unit. It is therefore a frequency, measured in "Hertz" ("revolutions per second") in the SI System.