Janka Hardness is good place to start, but not the only number
When deciding what species of wood I want to use in a project I will start with the color but then quickly look at the Janka hardness number to determine what the workability of that species might be. Its a great place to start to get you in the ballpark and to quickly compare it to other species you may have already used.
First the basics. The Janka hardness test measures the pounds of force (or Newtons) to press a .444" diameter steel ball halfway (.222") into the face grain of a board. You will find some variance in the numbers from one source to another and don't get too caught up in the exact number as they are all averaged over multiple tests and samples. You will find the hardness of the same species can change from one region to the next and one board to the next very easily. Wood is after all an organic material and subject to all kinds of external factors.
Just ask any tree downwind of the Mt St Helens eruption.
In this episode I will show that Janka hardness is often effected by other factors like porosity, density, extractive content, structure, or shearing strength which can make a wood feel harder or softer. Whether that wood feel harder or softer will sometimes change with the same factor too. Its frustrating but also why we all love to work with wood. Still considering the hardness is a great first step in determining how an unfamiliar species of wood will work with power and hand tools.