This symbol commonly worn as jewelry is simply the Hebrew word Chai life)The word "chai"has great significance to Judaism which is very focused on "life"'
In Judaism, the Chai symbol consists of the letters of the Hebrew alphabet Het (ח) and Yod (י). In the Hebrew language, the word chai (חי) spelled by these two letters means "living", and is related to the word for "life", chaim, and also appears in the slogan am yisrael chai (עם ישראל חי, "The people of Israel live!", referring to all Jews). There have been various mystical numerological speculations about the fact that according to the system of gematria, the letters of chai add up to 18 (see "Jewish use of the Tetragrammaton" and "Lamedvavniks"). For this reason, 18 is a lucky number in Judaism, and many Jews give gifts of money in multiples of 18 as a result.
The Chai symbol is often worn by Jews as a medallion around the neck (along with the Magen David or Star of David and the Chamesh).