Answer: to say a person is a heterozygous carrier of a disease means that they are a heterozygote for a recessive disease (Aa).
Say a disease has two alleles, A and a. “A” is dominant, and a person with even just one A would not display disease symptoms. “a” is recessive, and a person would two copies to develop the disease. A carrier would be the heterozygote, Aa. This person is considered a carrier because, while they do not have the disease themselves, they “carry” the ability to pass on the disease to their children if they have children with a person who is Aa or aa.
Hope this helps! Drawing some punnet squares of crosses between Aa x aa and Aa x Aa and looking at the occurrence of aa offspring may help as well.