The Eighth Amendment to the US Constitution is part of the Bill of Rights (the first ten Amendments). The Bill of Rights was added to the original Constitution to protect individual liberties from excessive governmental power. The Eighth Amendment in particular is intended to protect criminal defendants from unduly harsh penalties. It says, "Excessive bail shall not be required, nor excessive fines imposed, nor cruel and unusual punishments inflicted."
Bail is an amount of money that a released defendant deposits to ensure their appearance at trial. Whether bail is excessive is a matter of the judge's discretion. In 1951, the Supreme Court ruled that bail is excessive if it is more than is reasonably calculated to be sure that the defendant appears at trial.
Excessive fines would be financial penalties so large that they would deprive defendants of property without due process of law.
The most controversial aspect of the Eighth Amendment is the clause concerning unusual punishment, but this is extremely important. It protects prisoners from being tortured into confessing crimes and from punishments that do not fit the crimes. In the past, these coercive and punitive techniques would include the rack, thumbscrews, and gibbets. Nowadays, cruel and unusual punishments include those that are degrading to human dignity, arbitrary, unnecessary, or totally rejected throughout society.
Debate is ongoing as to whether the "cruel and unusual punishment" clause of the Eighth Amendment prohibits the death penalty. This is a continuing concern, as it is interpreted differently at federal and state levels as well as among various states. As you can see, though, the Eighth Amendment is of vital importance to ensure the rights of criminal defendants.