Final answer:
A multiplicative seasonality in models like the Holt-Winters method means that the seasonal effect changes in proportion to the data levels.
Step-by-step explanation:
A multiplicative seasonality, like in the Holt-Winters method, means that the seasonal effect varies with the level of the series. Instead of having a constant additive seasonal effect, a multiplicative seasonal effect implies that the impact of seasonality is proportional to the magnitude of the data. For example, in the context of sales data, if sales are typically twice as high in December compared to other months, the seasonal effect would amplify the forecasted sales by a factor that is relative to the overall level of sales.