The Janka hardness test measures the resistance of a sample of wood to denting and wear. It measures the force required to embed an 11.28 mm (.444 in) steel ball into wood to half the ball's diameter. This method leaves an indentation. A common use of Janka hardness ratings is to determine whether a species is suitable for use as flooring. The higher the Janka rating the harder the floor, which means its less likely to dent.

Janka Scale.png
 

Source: https://en.wikipedia.org/wiki/Janka_hardness_test