Normally when we get questions like these on genetics, it is helpful to know the genotypes of the two parents involved. I will start with Mendels work with pea plants, where he was breeding purebred tall plants that always produced tall plants and purebred short plants, that always produced short plants. One day he crossed a purebred tall with a purebred short, the result was all the plants were tall. So then, he crossed two of the resulting offspring of that purebred cross, and much to his surprise, 3 out 4 plants were tall, but 1 out of 4 were short! A Punnett square is helpful in explaining how the genotypes play out, if we use a"T" to represent the allele for tallness, and a "t" to represent the allele for shortness. Alleles are different forms of the same gene, in this case, one for tallness, one for shortness. That first cross of purebred tall (TT) and purebred short (tt) would have looked like this:
T Tt Tt
T Tt Tt
The phenotype of all the plants would be tall, because the allele for tallness is present in all of them. The recessive allele for shortness is also present, but the dominant allele trumps the recessive allele in physical presentation. The 2nd cross of the two offspring from the 1st cross (Tt) would look like this:
T TT Tt
t Tt tt
As you can see, 3 out of the 4 crosses have the allele for tallness; the first one is a purebred tall, while the other two are heterozygous tall, meaning they have different alleles present. The 4th cross has the reemergence of the recessive trait "tt", phenotypically is a purebred short plant. It should be noted that physical charicteristics that are recessive may only be expressed in the phenotype when they are purebred recessive.