A Punnett square is just a way to diagram a genetic cross. Suppose you are crossing two heterozygous individuals Aa X Aa. The Punnett square will look like:
Gametes |
A |
a |
A |
AA |
Aa |
a |
Aa |
aa |
This shows that 1/4 of the offspring will have the genotype AA, 1/2 will have genotype Aa, and 1/4 of the offspring will have the genotype aa.