It never was explained very well to me but I hate it and want it to go away
The gist of the thing is that people who are going to lend money to others want a way to know how likely they are to pay it back. A credit score is [theoretically] a measure of how reliable someone is with paying back debt. Thus, in order to increase it you have to take on debt and pay it back. Having a low credit score means that you don't have a history of paying back debt, so they ratchet up the rates to make up for the increased risk. On an individual level it doesn't make any sense, but from a higher level statistical viewpoint, the higher interest rates balance out the risk of nonpayment amongst a group.
The gist of the thing is that people who are going to lend money to others want a way to know how likely they are to pay it back. A credit score is [theoretically] a measure of how reliable someone is with paying back debt. Thus, in order to increase it you have to take on debt and pay it back. Having a low credit score means that you don't have a history of paying back debt, so they ratchet up the rates to make up for the increased risk. On an individual level it doesn't make any sense, but from a higher level statistical viewpoint, the higher interest rates balance out the risk of nonpayment amongst a group.