good explination. Here is a punit square

ill do this simple i'll use all a's capital A will represent normal and little a will represent a recessive trait (say albino)
mom's genes will be across the top, dads down the side
Mom will be homo for the trait, dad het
(ignore the dots, they're just to keep it lined up right)
...a ...a
A Aa Aa

a aa aa

The possible results are Aa, and aa
in the Aa ones the A (dominant trait, normal) covers the a (recessive trait, albino) . In the aa ones both genes are the recessive form, so the baby shows the trait. The Aa's cary the trait, and although they look normal can still pass the gene on (like the dad in this case)