Impressment was the act of seizing for public use or of impressing into public services. The British came overnight and secretly invaded the Americans ship. They threatened to shoot them if they didn't come with them. The Americans were forced to work for the British navy and did as they commanded,If they refused they would be shot.
The British were taking the American sailors hostage, and taking them aboard on the British Naval ships. Behind the Americans back, over and over again, night after night, the British stole the sailors.
The Americans kept losing sailors without realising, until they finally figured out that they were short on sailors. They realised that the British had been stealing from then all along.