A more thorough explanation:

A life-qualified jury is a group of people selected according to the law and given the power to decide questions of fact and return a verdict in a case submitted to them. In criminal cases involving the death penalty, a life-qualified jury is selected from a venire from which the judge has excluded anyone unable or unwilling to consider a sentence of life imprisonment instead of the death penalty if the defendant is found guilty.

For example, in a murder trial where the death penalty is a possible sentence, the judge may exclude potential jurors who are morally opposed to the death penalty. This ensures that the jury is made up of people who are willing to consider both life imprisonment and the death penalty as possible sentences.