USMC is short for the United States Marine Corps — one of the branches of the American armed forces. The Marines are trained to fight on land and from ships at sea, and they are often the first troops sent into a dangerous place, so Americans think of them as tough and proud. You see 'USMC' written on uniforms, caps, trucks, and signs rather than said out loud; when people speak, they say 'the Marines' or 'the Marine Corps', and each letter is said on its own (U-S-M-C), never as one word. Note the spelling of 'Corps': it looks like 'corpse' but the p and s are silent, so it sounds like 'core'. A person in it is 'a Marine', with a capital M — and Marines care about that capital letter. Outside the US military world this abbreviation is rare, so in normal writing it is kinder to your reader to write the full name once first.