A punnet is a small, low basket for soft fruit or vegetables, meant both to keep the items together, and 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. Sometimes they are referred to as “berry baskets.”
Punnets used to be made of wicker (more precisely, splint). Clerks at stores would empty a punnet’s contents into a small paper or plastic bag at the checkout, and retain the punnet for re-use.
Now, they are mostly seen made of single-use cardboard or plastic and regarded as disposable, and consequently sent home with the consumer.
1 punnet of fruit equals about 1 pint basket of fruit.