P1 cross -he crossed two pure (or homozygous) plants for a trait. For example, a tall plant was crossed with a short plant. He made sure they were pure by self pollinating them. P stands for parental.
The F1 or First Filial generation cross, two of the offspring from the P1 cross are the crossed. The recessive trait had seemed to disappear in the offspring of the P1 cross.
This is what Mendel could see (the phenotype)
The test cross is when we have, for example a tall plant and although we know tall is dominant we don't know if the genotype is TT (homozygous)or Tt (heterozygous). We do know a short plant is Homozygous recessive. So crossing the plant with the known will tell us what the genotype is.
50/50 tall to short and the unknown was Heterozygous.
All tall, the unknown was Homozygous Dominant