Yes, knowing what it is is the easy bit. It is % of the swept area actually occupied by the area of blades. High solidity would be a wind pump where the whole area is covered with blades. A low solidity would be like an AirX with narrow blades that only occupy a tiny part of the swept area.
High solidity rotors produce high torque but operate at low speed. At higher speed the next blade gets caught up in the wake of the first and the thing becomes inefficient, they have little tendency to over speed.
Low solidity rotors can capture the same power, but with low torque and high speed. The next blade is so far behind the first that the thing can run at a much higher speed without interference from the wake of the previous blade.
For pumping a high torque and self regulating speed characteristics are ideal for making use of low winds and still surviving high winds.
For electrical generation high solidity rotors need large speed increasing transmissions and are not cost effective compared with higher speed lower solidity rotors.
A high solidity rotor may have a tsr of perhaps 2 whereas the fastest rotors with tsr of 10 or more will need very narrow blades and a low blade count. A general compromise is for a tsr of about 5 to 7 which works out at a reasonable chord width for 3 blades.
The optimum tsr is influenced by the angle of attack as well as the solidity and for good results you need to get both factors about right.
Flux
