Organisms heterozygous for a recessive trait are often called carriers of that trait. what does that mean?

This means that those organisms have a recessive allele for a trait or mutation but does not display that trait or show symptoms of the disease. However, carriers can pass their recessive allele onto their offspring. If that offspring inherit another recessive allele, from another parent, it becomes recessive homozygous and displays that trait (or has the disease).