Even the slightest residue or "taste" of a non-kosher substence will render a food
not kosher. So it's not enough to buy only kosher food. The kitchen, too, must be "kosher," meaning that all cooking utensils and food preparation surfaces are used exclusively for kosher food, and that separate stoves, pots, cutlery, dishes, counter surfaces and table coverings are used for meat and dairy.
A general rule of thumb is that any time that hot food comes in contact with another food or a utensil, the food or utensil will absorb its "taste." Also cold foods and utensils will, under certain circumstances (such as when the food is spicy or salty, is cut with a knife, or it sits in the utensil for an extended period of time), transmit their "taste." So food prepared in a kitchen or plant in which non-kosher food is
also prepared will invariably become non-kosher as well (unless the imbedded
taste is first extracted from the untensils in a special "koshering" process).
For guidelines on how to set up a kosher kitchen or to "kosher" a previously
non-kosher a kitchen see Koshering Your Kitchen