The RNLI is a British charity that runs the lifeboat service: boats and crews that go out to save people in trouble at sea around the coast of the UK. The letters stand for the Royal National Lifeboat Institution. Though actually they also work around the coast of Ireland. Many of the crew are volunteers, and the money comes from donations, not from tax. You may see RNLI lifeguards on some beaches too. It is not the navy. It is also not quite the same as the coastguard: the coastguard is a government service that helps organise a rescue; the RNLI is the charity that often owns the lifeboats and sends the crew. Say the letters R-N-L-I.