A reversionary pension is an income stream you set up with your superannuation that automatically continues to someone else (generally your spouse) when you die.
In effect, it’s a way of telling the trustee of your super fund what you want to happen with your super when you die – you’re telling them you want your pension to continue but to be paid to your spouse instead. (For obvious reasons it doesn’t work if you haven’t started a pension yet – there’s no income stream to continue.)