" ...food had appeared out of nowhere, so that the five long tables were groaning under joints and pies and dishes of vegetables, bread and sauces and flagons of pumpkin juice."
-- Welcome Feast in Harry's Fifth Year (OP11)
There are laws of magic governing what you can conjure and what you can’t, such as Gamp’s Law of Elemental Transfiguration, to which food is one of the five exceptions (you can’t create food out of nothing) (DH29). However, food can be increased or transported from one place to another. All the food at Hogwarts is cooked by House Elves, then magically sent through the ceiling to the tables above (GF21).
References from the canon
When Harry and Ron arrived too late for the feast in third year after flying the Ford Anglia to Hogwarts, McGonagall conjured sandwiches and pumpkin juice to Snape's office (CS5). But it was not conjured out of thin air, and had to come from the Hogwarts kitchens.
When Harry, Hermione and Ron were on the run from the Snatchers, they had to procure food from shops or farms or go hungry (DH14, DH15).
Wizards are allowed to increase the food they have with the Engorgement Charm, as Hagrid does with vegetables on the Hogwarts grounds, such as giant pumpkins (CS7).
Ollivander used Viktor Krum's wand to create the Fountain of Wine spell (GF18).
Harry was able to conjure water using the Water Conjuring spell Aquamenti (HBP11, HBP26).
Harry learned that Tom Riddle's mother, Merope, didn't bother to get food through magical means when she was pregnant because she had given up on life when her husband left her (HBP13). Dumbledore said it was because she was "greatly weakened by long suffering."
Commentary
Notes
Molly Weasley is probably the Queen of Gamp's Law, stretching food to feed her brood of children, plus Harry, Hermione, and members of the Order of the Phoenix. Despite her family's alleged poverty, she offers Harry "fourth helpings at every meal," and for breakfast they eat "half a dozen bacon sandwiches each" (CS4). Molly could magically chop meat while making a sauce pour from her wand (GF5), while Arthur helped with chopping vegetables and meat for Molly's stew at Grimmauld Place (OP5). Note that the Weasleys never made the food just appear - they had to cook it.