In statistics, quantitative forecasting consists in making predictions about the future based on numbers acquired in the past.
You can estimate future values by identifying a data pattern in the past and continuing it, for example.
An example of a quantitative forecasting method used in sales statistics is the linear regression method. This involves calculating an average based on the data you already have about your sales in the previous months or years. You can then look at the way this average evolves and follow this trend as a forecast for the future.
In the attached graph, the average is the dotted line. It was calculated based on the data represented by the blue spots. You can see that the line extends in the future; this is your forecast.