Hi, I'm Mihai Chereji, from Cluj, Romania.
Sound, as the human ear perceives it, is basically a (longitudinal) wave that propagates through air (well, other materials as well, but let's stick to air for now).
Frequency is the property that measures how many times per second an event occurs. When talking about sound, the event being measured is how often air is compressed. Frequency is measured in Hertz (Hz for short). 1 Hz = 1 occurrence/second.
Frequency is perceived by humans as pitch. The higher the frequency of a sound, the higher the pitch that a human ear will perceive it as.
Human hearing is limited between approximately 20Hz and 20.000Hz or 20 KHz), but the higher end of the spectrum tends to go down with age, when the eardrum becomes less elastic. (sidenote: this has been weaponized by shopkeepers against loitering teenagers and used as a students-only ringtone - I'm 25 and can still hear it, yeah!)
The human ear is especially adept at hearing between 2KHz-5Khz (which are, not coincidentally, the frequencies around which the human voice are centered on), but as the amplitude of the sound increases (at around 80-100 dB SPL), the perceived loudness of sounds at any frequency tends to flatten out.
This is visible in the so-called Fletcher-Munson curves (now renamed as equal-loudness contours).