Answer:
A metronome is a device for measuring or setting the tempo, or speed, of a piece of music. As such, terminology such as "high" or "low" are confusing because such words typically refer to pitch.
Instead, one might indicate a tempo being "fast" or "slow".
Tempo is measured in "beats per minute", or bpm. The metronome was invented in the early 19th century, and famously used by Beethoven to specify "authoritative" tempo markings for his compositions. Prior to the invention of the metronome, composers would indicate the speed by Italian terminology such as "Allegro" (fast), "Allegretto" ('little' Allegro), "Presto" (very fast), "Adagio" (slowly), "Largo" (very slowly).
Once the metronome came into being, many of these Italian words took on a numerical value as well. An "Allegro" tempo is generally assumed to be in the range of 116 to 136 bpm. "Presto" would range above 160 bpm. "Adagio" can be closer to 72 bpm, whereas "Largo" might be as slow as 54 bpm.
A good musicians can learn to judge tempo by watching the second hand of a clock (which ticks away at precisely 60 bpm).
Most mechanical metronomes will top out around 180 or 200 bpm. Some digital metronomes will run all the way to 255 bpm.