Seasonality in time series occurs when the time series shows a repeated pattern over time. E.g., stationery sales decreases during the holiday season, air conditioner sales increases during the summers etc. are few examples of seasonality in a time series.
Seasonality makes your time series non-stationary because of the average value of the variables at different time periods. Differentiating a time series is generally known as the best method of removing seasonality from a time series. Seasonal differencing can be defined as a numerical difference between a particular value and a value with a periodic lag (i.e. 12, if monthly seasonality is present)