What is inverse mass? How should I determine it?

[Snail921]

Hi.
I create obi actor objects by script in run time without setting actor.solver.invMasses[index] for each particle. It seems the obi handle everything and give some proper parameters to my actors but I do not understand what inverse mass is and why we use it. I googled but could not find proper document that I could understand.
For now I have no issue with that but I would like to have better understandings for the future.
In a ObiActorCenterOfMass.cs, I found a line below:

• massAccumulator += 1.0f / invMass;
  

I understand above line as below:

• Mass = 1.0f / (Sum of inverseMasses)
  

For example, if I would like to make an actor with four particles with a total mass of 150 it should be like this.

• 150 = 1 / (4 * invMass)
  
• invMass = 1.0f / 600f = 0.00166
  

The answer indicates I give 0.00166 to each particle's invMass.
Someone please let me know if I am correct.