In every culture there is the basic ingredients that are used in many dishes. Due to there frequent use people keep a lot of them in their kitchens. These are refereed to as store cupboard ingredients. Also the ingredients usually have long shelf lives and therefore can be kept for long periods of time until they are needed and people can always make sure they have them in the cupboard. They are great for when you have nothing in the fridge to cook and you can whip them together to create a decent meal (see my Spinach and Roccitta Pasta Bake recipe for what I made with them).
For Italians these 'store-cupboard' ingredients are generally; pasta, rice, tomato sauce, pesto and nutmeg. However as you may know cooking varies across Italy in the different regions so much that for each region these could be different.
In the fridge there is also always Parmesan and ricotta cheese. And the freezer always has frozen spinach. These ingredients can be combined in different ways to create many different dishes that taste insanely good. The Italians know very well how to take a few simple ingredients and turn them into a multiple course meal that tastes amazing!