A red hammer and sickle. It was the winner of a competition in 1917, and officially adopted by the Soviet Union in 1923.

A sign is something you see (or hear) which means something. A sign may be natural or man-made.

A natural sign shows something significant. Leaves turn brown and fall off deciduous trees in the autumn. Temperatures drop, daylight hours shorten. It suggests winter is coming. Many birds migrate and some mammals hibernate in response to natural signs like these. Humans, as usual, are a bit different. They make their own signs.