It is supposed to represent the shape of King David's shield (or perhaps the emblem on it), but there is really no support for that claim in any early rabbinic literature...

In the 17th century, it became a popular practice to put Magen 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; however, I have never seen any explanation of why this symbol was chosen, rather than some other symbol

The Magen David gained popularity as a symbol of Judaism when it was adopted as the emblem of the Zionist movement in 1897, but the symbol continued to be controversial for many years afterward. ..