Test cross is when you cross (aka. "mate" in simple language) the particular pea plant (which you would like to find the genotype of) with a pea plant which is homozygous recessive.
Let's say allele T codes for the protein which synthesizes the growth hormone which makes the pea plant tall. Allele T is dominant to allele t, which does not code for extra growth hormones when in homologous recessive form (tt) as allele T does.
Tall pea plants may have any of the 2 genotypes as follows: TT, Tt.
The test cross of TT:
TT × tt will give all Tt offspring, which satisfies the criteria in the question, where all of the offspring via the test cross are tall.
The test cross of Tt:
Tt × tt will give Tt, tt, a phenotypic ratio of 1 Tall : 1 Short.
Thus, the genotype of the original tall pea plant is TT.
Hope this helps! :)
