"Y" = yellow ||| "y" = green ||| "R" = round ||| "r" = shriveled

What does this "Punnett square" stuff mean?

Figure 2.

Figure 1.  "Purebred cross:" Alleles Y and R and y and r will always segregate together (there is no alternative) yielding a hybrid first filial (F1) generation (children).  [genotype variation: 1; phenotype variation: 1]
Figure 2.  "Independent Assortment:" A dihybrid cross. 4 Classes of gametes yield genotypic variation of 9:3:3:1 and phenotypic variation of 9:3:3:1
Figure 3. "Dependent Assortment:" 2 Classes of gametes restrict genotypic variation to 3:1:1

A "punnett square" according to merriam-webster is "an n x n square used in genetics to calculate the frequencies of the different genotypes and phenotypes among the offspring of a cross."