It means company has issued bonds, normally they pay interest periodically on those, if it is a zero coupon bond then no interest is paid during the life of the bond but to compensate for that those zero coupon bonds are issued at a deep discount and redeemed at much higher price or face value upon maturity.
Convertible bond means after few years of issue those bonds are converted to equity shares, the ratio will be predefined and clauses will be predefined to allow conversion.
Zero coupon compulsorily convertible bond has features of both zero coupon bonds and convertible bonds. So, in this case the same has happened, I hope you got it.