What does it mean when traders are indifferent?

[theBEAST]

I have attached a picture of the question that deals with taxes, supply and demand curves and tax revenue.

I don't understand what the question is asking. There is another version of this:
"suppose that traders who are indifferent between trading and not trading do not trade. What is the predicted tax revenue for the government?"

I don't understand what the different between the two is.

 

[JonDE]

I have attached a picture of the question that deals with taxes, supply and demand curves and tax revenue.

I don't understand what the question is asking. There is another version of this:
"suppose that traders who are indifferent between trading and not trading do not trade. What is the predicted tax revenue for the government?"

I don't understand what the different between the two is.

I believe what they are meaning by "indifferent" is neither making or losing money by trading. Although that graph is a little hard for me to understand as it doesn't make much sense. Maybe I am looking at it wrong?