The frequency defines how high or low you perceive a tone.
How high or low you perceive a tone depends on how fast the cycles repeat. This speed is called frequency and is measured in the number of cycles per second. This unit is called Hertz (named after the German physicist Heinrich Hertz), but usually, you only see the abbreviation Hz.
If you read, the frequency is e.g. 440 Hz, it means there are 440 cycles per second. Higher numbers, and therefore higher frequencies are higher tones. Lower frequencies with lower numbers are lower tones.
Humans usually perceive frequencies between 20 Hz and 20 kHz (20'000 Hz). Be aware, “usually” means that while most humans have hearing in this range there are people that can perceive lower and higher frequencies.