There are two measures of hardness referred to in regards to finished timber products. These are abrasion & indentation resistance.
Abrasion is a direct function of the finish coat/s, their hardness as well as their ability to flex.
The usual measure of the ability of the substrate to withstand indentation is the Janka Hardness Test. This measures the force required to embed an 11.28 mm (0.444 in.) steel ball to half its diameter into the wood, thus forming an indentation with projected area of 1 sq. cm.
The Force is expressed in Kilo-Newtons (kN). 1kN = ~100Kg, thus a figure of 5kN means that it requires 500Kg to make 1sq.cm2 indentation that is nearly half a family car! This is more a critical factor when deciding on a species for timber flooring, (where a stiletto on dance floor can make deep indentations on softer timbers) than for, say, kitchen tops.
Kiln Dried timbers have Janka hardness ranging from c.1.5kN for some species of Spruces and Firs to 15.0kN for some exotic species like Wandoo in Western Australia.