What you (or the company) would do is calculate the average change in annual sales of the product over, say, the last 5 or 10 years. Say it looks like on the average that sales grow by 7% a year. This is the trend - an 7% increase. We then assume that annual sales of the product in the following years will show an 7% increase from year to year. Notice that this has a compound effect and that it predicts that in ten years the demand for the product will roughly double.
This is a very basic description - the actual process is much more sophisticated. For example, you might want to weigh the most recent years more heavily in your calculations