Sacrifices of various kinds, including human sacrifice, were an integral part of Aztec culture. Though by no means the only ancient civilization to engage in the practice, the Aztecs have gained an especially fearsome reputation throughout history for their penchant for human sacrifice. Without in any way trying to minimize the moral degradation involved, it's important nonetheless to understand why the Aztecs carried out what to civilized people everywhere is an utterly abhorrent practice.
In Aztec culture, human sacrifice was seen as a way of repaying the gods for the sacrifices that they themselves had made in creating the world; it was an offering of thanks, an expression of gratitude for all that the gods had done and continued to do. The gods were all-powerful, but still needed to be nourished with a regular supply of sacrificial victims' flesh and blood in order to have the strength to carry out their vital functions such as making the sun rise.
According to the Aztec creation myth, the great gods Quetzalcóatl and Tezcatlipoca created the earth and the sky, all the mountains, streams and rivers, from the body parts of the giant reptile monster Cipactli. To appease Cipactli for the loss of her limbs, the gods promised her human hearts and blood by way of compensation. Again, one can see from this that Aztec human sacrifice rituals expressed the reciprocal relationship between gods and mortals.
There were many different kinds of human sacrifice in Aztec culture; some involving the ritual killing of children, whose tears were believed to herald the prospect of rain to fertilize crops; others involving warriors from rival tribes captured in battle. However, whoever the victims were, Aztec human sacrifice was primarily concerned with maintaining an appropriate balance between mortals and gods, in which the two worked together to ensure the harmony of the created order.