If T is for the dominant allele of a trait (eg. phenotype of brown fur) and t is the recessive allele of a trait (eg. phenotype of white fur) then a heterozygous dominant genotype for this trait would be Tt .
Heterozygous means both alleles are different (so Tt not tt or TT)
A genotype is what the genes say (one allele for brown fur one allele for white fur) while a phenotype is what the organism displays (in this case brown fur because the allele is dominant and therefore only needs one allele present for this trait to be displayed)