That is the inductance of the coil in milli-Henrys.
It is a combination of the core and number of windings. The higher the Henery number the slower the current can change, but it should create a larger magnetic field which can produce more energy.
Coils with larger inductance values are used with conventional ignition systems, because the spark is created by the collapse of the magnetic field.

I found this link interesting:
http://linuxfan.org/~ipdown/mybrick/automotive/IgnCoilAnalyzePrez.pdf