The number beside each piano key is the fundamental frequency in units of cycles per seconds, or Hertz.
For example, the A4 piano key has a frequency of 440 Hz.
Note that A5 has a frequency of 880 Hz. The piano A5 key is thus one octave higher than A4 since it has twice the frequency.
An overtone is a higher natural frequency for a given string. The overtones are "harmonic" if each occurs at an integer multiple of the fundamental frequency.