I think it can take two meanings.
The first meaning is in keeping with the Quaker use and maybe typified by Shari Runner in this Huffington Post article:
Speaking truth to power means believing deeply in what you say and fighting every day to have that heard.
It may not be popular; it means taking a risk, it means standing for something.
But depending on the context, it has a second, less grandiose meaning that I can best describe like this:
“Speak truth to power” means speaking what we believe to be true to someone in authority who might take it as a criticism or be offended and who has the power to punish us in some way.
In writing my blook, I am researching the barriers to what makes a good conversation and being afraid to "speak our minds" is clearly one of the obstacles. There are many reasons why we might be reluctant to "speak up." Fear of authority is one of them.
But I am starting to use the phrase in a slightly different way. Fear comes from the power difference between ourself and the other person. We feel we have less power (perceived or real) than them and so are afraid. But there are many forms of power difference:
What I am trying to better understand is how we learn to "speak truth to power" in all of these contexts or create conversational environments that make it easier.
I welcome your thoughts :-)