What is impressment and why do the British consider it okay to impress American citizens?

Impressment refers to the practice of forcing seamen into service for the British Navy (primarily, although others did this).

The general thinking before the Revolution was that the colonies existed for the benefit of the Mother County (Britain) and that if the British Navy was being used to supply, support, and protect the colonies than the British Navy had the god given right to choose sailors at will. This was often accomplished by capturing drunk sailors at bars.