If you are asking about 'tension in the negative spaces'...
it refers to the contrasts that are created by light and shade,
the balance between black and white,
the interest that's created by spacial relationships...
It's often a term that relates to art, and how things react with each other
or excite each other depending on how the artist uses space and form...
it's about what is shown, or not shown (negative)
that builds a tension for the viewer...
it's a bit of a play on words that for me means life has lots of grey areas
(it's not all 'black and white'.)
You can't always understand or resolve things easily,
and sometimes the best thing to do
is to somehow hold the tension between what could be seen
as opposite concepts...or emotions.
The 'quiet heart' tries to highlight the idea
that in order to be at peace (in ones inner being/heart)
one has to negotiate the depths and heights of emotion and experience.
It's not necessarily good or bad,
it's the type of thing that life is full of...
However, being able to hold those opposing emotions
in balance is the important thing.
I guess at the end of the day
it's refering to accepting life's' contradictions with grace.