Meaning
A cross with three bars: a short one at the top, the main crossbar and a slanted bar near the foot. It is the cross of Eastern Orthodox Christianity, and the slanted lower bar is its most recognisable detail.
Orthodox Christians use it in prayers, bios and greetings for feast days such as Orthodox Easter and Christmas: “Christ is risen ☦️ happy Easter to all” If you simply mean Christianity in general, the plain ✝️ is the more familiar choice.