The American merchants were taking their cargo by ship to trade with foreign places. This was before the impressment during the Napoleon War.
The British knew they were low on man power so they had to do something since they were at war. So they decided to kidnap American sailors at sea with U.S.A not knowing. That was the Impressment in the 1800s. There plan did not work out though and U.S government found out about this. In the photo you can see the sailors being caught by the British
This photo explains the sailors moving from their ship into the British one being captured and forced to go to the British Navy or die. Since U.S found out about this it ended up being another war between America and Britain. Soon the war was over and so was the Impressment.