"Kitchen string" refers to a type of strong and heat-resistant string used in cooking and food preparation. It is typically used to tie up meat, poultry, or other ingredients to hold them together during cooking. Full definition
Using kitchen string, tightly tie the lamb in several places (see photo above). (cookincanuck.com)
Tie the carrot, celery, lemon, bay leaf, parsley stalks and rosemary together with kitchen string and put into the turkey cavity. (supergoldenbakes.com)
Stretch a good length of kitchen string along the work surface and overlap the rosemary along the length of string. (bbcgoodfood.com)