Impressment was the practice of forcibly inducting men into military service.
In England, impressment was historically employed by the army and navy, but by the 19th century it was commonly used only by the navy. From the British viewpoint, impressment made some sense. Conditions in the Royal Navy were extremely harsh, resulting in a high number of desertions.
Britain at the time did not recognize the process of renouncing citizenship once an Englishman, always an Englishman. Therefore, they felt justified in taking any of their citizens found on the ships of other nations.
The impressment issue grew to crisis proportions during the Napoleonic Wars in the early 19th century. Britain needed all the men that could be found and was often careless about whom they impressed.
Often it amounted to taking any able-bodied English-speaking man. The American government tried to help its seamen by providing them with documents attesting to their American citizenship, but these papers were largely ignored by the British.
Impressment did not always work well for the British. It tended to fill her crews with unhappy men who were primed for mutiny. The practice largely died out after the defeat of Napoleon.