The Gross Domestic Product s a measure of the total value of all goods and services produced by Germany.
A high reading or a better than expected number has a positive effect on the EUR, while a falling trend is seen as negative (or bearish).
The GDP is considered as a broad measure of the German economic activity and health.
There are 2 versions of GDP released about 10 days apart - Preliminary and Final. The Preliminary release is the earliest and thus tends to have the most impact.