The TT parent can produce only T gametes, and the tt parent can only produce t gametes.
The TT genotype means that the individual has two copies of the big T allele for that gene (stem height). When gametes are produced, one of those two alleles will go into each gamete. Since both alleles are identical in this case, this individual can only produce gametes with a big T.
The tt genotype is the same, but both copies of the allele are of the recessive variety.
In this cross, all the offspring will be heterozygous (Tt) and tall, since the tall allele is dominant to the dwarf allele. A Tt individual could produce two different gametes, T and t. But in the question you posed, the TT parent can produce T gametes and the tt parent can produce t gametes.