Wat is Conversion premium?

Example 21.8 (The convertible across share prices)

Under a constant 5% hazard, the convertible is worth 89.1, 100.2, 116.0, 156.7 and 203.2 at share prices of 20, 30, 40, 60 and 80. Its bond floor is 83.2 throughout. The conversion values are 50, 75, 100, 150 and 200, so the conversion premium falls from 78% at 20 to 34% at 30, 16% at 40, 4.5% at 60 and 1.6% at 80. The delta rises from 0.84 shares per bond at 20 to 2.39 at 80, approaching the conversion ratio of 2.5.