What does this scale mean?
To expand upon the comments, the y axis in this graph is simply the log of the concentration in pg/ml.  To convert you use the equation $$C = 10^{y}$$ where y is the y value and C is the concentration.  Note that in older literature, log was oftentimes assumed to be base e whereas nowadays we assume base 10.  The figure (thankfully) clarifies this potential discrepancy.

A final note: log values are unitless so the y axis has no units.  The units are reported here so that you know what the appropriate units of the converted value should be.