A bond is an item that can be used to purchase 14 days of membership. They can be bought in the Grand Exchange, they can be traded, and you can buy them off of their site for IRL money. Hope I helped you out.
A bond is basically an in-game item that you can buy with $ or RSGP and can be redeemed for 14 days membership, 8 sof spins, or 120 runecoins I believe.