If you have a trait determined by 1 gene with two alleles you have one dominant allele and one recessive allele. Dominant alleles are represented by capital letter and recessive alleles by lowercase letters. You can have the following combinations
FF - Homozygous dominant
Ff - Heterozygous dominant/recessive
ff - Homozygous recessive
Homozygous means that the alleles are both the same. Heterozygous means that there is one dominant allele and one recessive allele.
The above are examples of genotypes. Each genotype produced a phenotype. A phenotype is the trait that is being expressed.
Let's use the example of wrinkled peas being recessive and smooth peas dominant.
Genotype FF = Phenotype Smooth Peas
Genotype Ff = Phenotype Smooth Peas
Genotype ff = Phenotype Wrinkled Peas