The recessive allele is said to be an allele whose effect is masked by the dominant allele of that gene.
In a heterozygous pair, only the dominant gene allele is expressed.
This recessive allele is expressed in the phenotype only if it is homozygous, which means a pair of the recessive allele is present on the homologous chromosomes.
In Mendel's experiment, he crossed homozygous tall pea plants with dwarf pea plants and he noticed in the first generation all were tall plants, which were heterozygous in nature but the recessive character was masked phenotypically.