A Punnett square is used to find out the possible genotypes of the next generation after a cross from 2 parents.
Usually, the letters outside the box are the genotypes of the parents, and the letter inside is the possible genotypes of their next generation.
The image inserted is an example of a Punnett square (made in MS word). It shows the cross of parents with genotypes Aa, and thus leads to possible next generation with genotypes of either AA, Aa or aa.