In Six Sigma, Xs are the terminology for the input and process variables in your process, such as the temperature in the oven when cooking a meal, or the length of time it takes to fulfill an order. These are the sources of variation in the process. In contrast, the outputs of the process are indicated as Ys, such as the temperature of the meal when it is served. Hence you will see the reference to Y=f(x). This simply means the output (Y) is a function of the input and process variables (Xs). In this example, the temperature when the meal is served is a function of the cooking temperature and the length of time it takes to fulfill the order.