What does it mean to be a carrier

Because Fragile X is carried on the X chromosome, which is one of the sex determining chromosomes, female carriers have a 50% risk of having a child, male or female, with the premutation (a carrier) or the full mutation.

A male carrier will pass on the premutation to all of his daughters as they will all inherit his affected X chromosome, but this expansion will remain stable and so none of his daughters will become full mutation.

He can not pass on Fragile X in any form to any of his sons as his male children receive his unaffected Y chromosome.