A punnet is a small, low basket for soft fruit or vegetables, meant to protect the items in it during shipping and retailing.
It is a term used in most of the English speaking world outside of North America. In North America, they are often seen as small, green plastic square baskets holding berries or cherry tomatoes. North Americans just never knew the word for what they were looking at.
Punnets used to be made of wicker (more precisely, splint), but now are made of cardboard or plastic, and are single-use only.
Equivalents
1 punnet of fruit equals about 1 pint basket of fruit.