Hi,
A nevus is a skin lesions commonly referred to as a "mole". An "atypical" or "dysplastic" nevus is a mole with some characteristics different from a normal or typical mole.
There is some risk that a dysplastic nevus may give rise to a melanoma. Although your wife has no need to panic, close surveillence is definitely warranted.
You may want to discuss with your dermatologist or surgeon if he/she would like to repeat the surgery to excise the remainder of the nevus as the pathology report has found one margin positive.
All the best, and God Bless!