Impressment was the act of taking men into a military or naval force by compulsion, with or without notice. The British were doing this to the American sailors when they were at war with France
At first, the British navy had impressed men off of British ships and out of British port towns. This was a legal thing to do to British people. When the British began to impress the American sailors, they were treating them as if they were still British subjects.
British impressment of American sailors was one of the major causes of the War of 1812. It helped to cause the war because it was made clear the fact that the British did not respect American rule.