A carrier is someone who has a disease-causing variant (or change) in one copy of a gene.
For autosomal recessive conditions, a person must have two disease-causing variants (one in each copy of the gene) to be affected by the condition. Carriers of autosomal recessive diseases are usually healthy, but may have an increased risk of having a child with a genetic disease.
X-linked conditions affect men and women differently. Women who have one abnormal X chromosome are usually healthy carriers, but have an increased risk of having a child with the condition. Men who inherit one abnormal X chromosome are generally affected by the condition.