The Star of David, Jewish Star, or Magen David means Shield of David. It is one of the most popular Jewish symbols that has been in use at least since Babylonian times.
As popular knowledge, it is said that the young Israelite lad, David, who would later become King David, used a shield in this shape when he went to battle.
Use of Star of David in Kabbalah
In Kabbalah, the Star of David symbolizes the six spaces of space plus the center; Up, Down, East, West, South, North and Center.
These are also reflected in the seven spiritual building blocks (Sefirot) with which God created the world: Chesed (Kindness), Gevurah (Severity), Tiferet (Harmony), Netzach (Perseverance), Hod (Splendor), Yesod (Foundation) and Malchut (Royalty).
Another kabbalistic meaning is stated in the Zohar:
There are three knots connecting one to another, The Holy One, blessed be He, Torah and Israel. One triangle represents this 3 sided connection on an inner dimension, and the second triangle refers to an external demonsion. The essence of the soul connects with Gods essence through the study of Torah and Kabbalah. The external connection to God is through practical application of these studies.
The number seven has multiple significance in Judaism for example;
Another idea is that a six pointed star receives form and substance from its solid center. This inner core represents the spiritual dimension, surrounded by the six universal directions.
According to G.S. Oegema –
“Isaac Luria provided the Shield of David with a further mystical meaning. In his book Etz Chayim he teaches that the elements of the plate for the Seder evening have to be placed in the order of the hexagram: above the three sefirot “Crown”, “Wisdom”, and “Insight”, below the other seven”. 
Similarly, M. Costa wrote that M. Gudemann and other researchers in the 1920s claimed that Isaac Luria was influential in turning the Star of David into a national Jewish emblem by teaching that the elements of the plate for the Seder evening have to be placed in the order of the hexagram, but Gershom Scholem proved that Isaac Luria talked about parallel triangles one beneath the other and not about the hexagram.  The Star of David at first was only known remotely and would not be considered ‘official’. At the time the nearest thing to an Islamic symbol would be a menorah.
The six days of Creation and the seventh day of rest.
In the 17th century, it became a popular practice to put Star of Davids on the outside of synagogues, to identify them as Jewish houses of worship in much the same way that a cross identified a Christian house of worship.
During the Holocaust, the Nazis forced the Jews to wear a yellow Star of David as a method of identification.
Today, in the State of Israel, the Magen David Adom (red Star of David) symbols official emergency medical ambulance service. It is an official member of the International Committee of the Red Cross.
Wearing Star of David Pendants is a popular way of expressing identification with Judaism.