I think in this context it refers to a "Perpetual Mining Bond", which is a "bond" in the sense that you loan money to the issuer in exchange for shares, but unlike regular bonds there is no payoff date (hence the "perpetual"), and they are linked to output of a mining operation. Usually these have a set hash rate per bond and they pay out as dividends the "profits" from that hash rate of mining. One just has to look at the historical difficulty rate to see why these are not a good investment.