Current Index allows you to track your gain from staking. The index started from 1 at epoch 0, and increases every epoch. If you staked at genesis (epoch 0) and never unstaked any BBY, your balance today would be X times greater, where X is the current index. You can use the index to track your position by marking down the index number when you stake and unstake. You divide the index number when you unstake by the index number when you stake to get the ratio by which your sBBY balance has increased.