For someone to develop the symptoms of an autosomal recessive disease, it is necessary that he or she has both faulty copies of the affected gen. If only one of the defective copies is inherited, the child will be a carrier of the disease. Being a carrier means that one does not have symptoms of the disease, but can transmit it to offspring.
For instance, given the case of a family where both the father and the mother are carriers, the chances of having a healthy child are 25%, whilst the likelihood of having a carrier child are 50%. Finally, there would be a 50% chance of having a sick child.