The number of times and event occurs over a given period of time (usually one second). In most cases frequency relates to a regularly occurring event of a cyclic nature. Frequency is measured in Hertz , Hz, which is the SI unit defining cycles per second. It is named after the German physicist Heinrich Hertz who was not the founder of a car rental company we understand , A 440Hz tone describes the pitch of A4 audible tone. In electronic terms we often talk in terms of MHz, 10^6, and GHz, 10^9, cycles/second. For example, a specific frequency defines the clock rates in digital systems. Here are some of the more common ones used in TV:

PAL subcarrier: 4.43 MHz
NTSC subcarrier: 3.58 MHz
ITU-R BT.601 luminance sampling rate: 13.5 MHz (SD)
ITU-R BT.601 chrominance sampling rate: 6.75 MHz (for 4:2:2 SD sampling)
ITU-R BT.709 luminance sampling rate: 74.25 MHz (HD)
ITU-R BT.709 chrominance sampling rate: 37.125 MHz (for 4:2:2 HD sampling)
ITU-R BT.2020 luminance sampling rate: 297 MHz (4K UHD)
ITU-R BT.2020 chrominance sampling rate: 148.50 MHz (for 4:2:2 UHD sampling)

Although not appearing in any prominent headline, 2.25 MHz is significant as the greatest common divisor multiple of all these frequencies, meaning they are all related.

See also: 13.5 MHz