Impressment is basically when the British came to America and secretly took away American sailors without their permission. The Americans did not notice what had happened to their sailors
The reason why the British took the American sailors is because they were in a massive war with France and they needed sailors, so since before they owned America, they just snatched the sailor and kidnapped them onto their ships
So what they did to the sailors was just sail them away with their hands tied with a rope behind their back and take them from ship to ship until they get to Britain for war against the French.