That yellow police tape and group of officers encircling the crime scene? That's a kind of cordon — something set up to guard something.
A cordon can also mean some kind of ornament or adornment made out of ribbon or cord, usually worn by military folks to signify honor or rank. Not surprising since cordon sounds similar to cord and comes from the Old French word for the same thing. But unless you're part of a security team or often dressed in full military regalia, you probably won't come into contact with cordons very often.