Short answers, pulled from the story.
The Berlin Decree was issued by Napoleon on the 21st of November 1806 from Berlin, shortly after France's victory over Prussia at the Battle of Jena. It declared the British Isles to be in a state of blockade, prohibited all commerce and correspondence with Great Britain, and ordered the arrest of British subjects found in French-controlled territory.
Napoleon's Continental System aimed to force Britain's government to the peace table by cutting Britain off from European trade and wrecking its economy. The strategy depended on denying Britain access to continental markets and seizing British goods and subjects throughout French-controlled Europe.
Britain controlled the Atlantic Ocean trade routes, which allowed it to redirect commerce to markets beyond Napoleon's reach. Historian Paul Schroeder considers the Continental System to have been an ineffective method of economic warfare, and enforcing the blockade across Europe's vast coastline proved impossible.
Any vessel that had called at a British port or a British colonial port and then docked at a continental European port was to be treated as British property. Both the ship and its entire cargo were subject to confiscation.
The Milan Decree was issued by Napoleon the year after the Berlin Decree, in 1807, and served the same purpose of enforcing the Continental System against Britain. It extended and reinforced the framework Napoleon had established with the Berlin Decree.
The Continental System created economic hardship across France and its allied nations, making it unpopular among French subjects and allies. Other European nations withdrew from the system, and Russia's departure was among the causes of Napoleon's invasion of that country, a military catastrophe that contributed directly to his fall.