Sex-determination in humans is dependent on the sex chromosomes. Females are XX. Males are XY. Since males have only a single copy of the X chromosome, they have only a single allele for any gene on the X chromosome.
Males are said to be "hemizygous" for any X-chromosome genes, meaning that there are only half ("hemi") as many alleles as normally present for a diploid individual. As a consequence, if males (but not females) inherit an X-linked recessive trait from their mothers, they display the recessive phenotype.