The internal length units per meter.
The physics system bases all length units on meters but you may need different units for your project.
You can set this value to use different units but it should only be modified before any other calls to the physics system occur and only modified once.
Changing this value after any physics object has been created can result in severe simulation instabilities.
Essentially there are some internal tolerances, such as how close two shapes need to be before they are considered to be touching or when two vertices of a hull are so close that they should be considered the same point.
For example, internally a value of 5mm (0.005 meters) is used as a value tuned to work well with most situations with game-sized objects described in meters.
If you decide to work in a different unit system (such as pixels) then 0.005 pixels is not a good value for this constant and would be too precise, leading to numerical problems, especially far from the origin.
Instead you should determine roughly how many pixels you have per meter. For example, say you want 32 pixels per meter then you should set the lengthUnitsPerMeter to be 32.0f.
Setting a value of (say) 32.05 would result in the 5mm being scaled up to 0.16 meters, which is a more reasonable value for determining if shapes are touching and hull vertices are too close.
A good rule of thumb is to pass the pixel height of your player character to this function, so if your player character is 32 pixels high, then pass 32 to this function.
Then you may confidently use pixels for all the length values sent to the physics system.
All length values returned from the physics system will also then naturally be in pixels because the physics system does not do any scaling internally, however, you are now responsible for creating appropriate values for gravity, density, and forces.