Seasonality is fluctuation in sales figures throughout the year due to varying demand.
Some products are more impacted by seasonality than others - coffee is bought more regularly than swimsuits, for example. Also, ecommerce as a whole experiences seasonality: the holiday season (November and December) are the busiest months, while the time of the summer vacations in the Northern Hemisphere are the slowest period.
One way to fight seasonality and get a more evenly distributed revenue is to offer a range of products that cover each other’s slow times. Another is to maximize sales when you can so you don’t run into negative cash flow later.