What you are referring to is a chart from Azure Photonics which they call an MTF chart.
It says (see source): Distortion : <12~1% (not <12-1%)
I believe it is saying that within 12mm of the optical centre the distortion is about 1%
<12 I take to mean "within 12mm", and ~1% means "about 1%"
The chart measures the % distortion (of horizontal or vertical lines, how much they would bend as a percentage of the overall image height).
I believe the graph shows the centre of the lens (0mm) at the bottom, moving up to 10mm (or 100% of the lens, I'm not sure) at the top. The x axis is the distortion, so it is approaching 1% at the top of the graph and about 0% at the centre of the lens.
The distortion is negative, which seems to indicate barrel distortion, as opposed to pin cushion distortion
More info on distortion and interpreting MTF charts: digilloyd