A basket of apples are obtained by adding 1 to 5 cooking apples into a basket. This is a convenient way to carry them and reduces used inventory space. Like all baskets, it has the same functions, such as "remove one" and "empty".
A basket of 5 apples is used in the Farming skill as payment to pay a farmer to protect strawberries in an allotment patch or Willow trees in a tree patch. Four baskets of apples are used as payment to grow Banana trees in the Farming skill. Payments to farmers can be noted, but loose items cannot be used if baskets of items are requested. The baskets must be full.
Apples, individually, are called cooking apples in-game. Cooking apples can be purchased from the Culinaromancer's Chest after starting the quest Recipe for Disaster, picked from apple trees, found at the Cooks' Guild or thieved from Fruit Stalls in Kourend; just south of the Xeric's talisman teleport to Xeric's glade.