Summary

jerusalem cross pendant
The Jerusalem cross was used as a symbol of the crusader Kingdom of Jerusalem in the 12th and 13th centuries. In heraldic terms it's a "cross potent" (a cross with, well, literal crossbars at the end of each bar), surrounded by four smaller Greek crosses (the sort of cross you see on the flag of Greece, naturally, but also Switzerland, or the Red Cross - the flag of the country of Georgia also has something reminiscent of a Jerusalem cross, but it's not exactly the same). Typically the crosses are gold on a silver or white field, which breaks the usual rules of heraldry where you can't have a metal on top of another metal...but those are much later artificial rules. The rules were much more fluid in the 12th century when these and other early coats of arms were being created.

Anyway, even after Jerusalem was conquered by Saladin in 1187, and the Kingdom of Jerusalem was fully destroyed in 1291, people continued to use these arms if they had a claim to the kingdom or they wanted to depict themselves as crusaders. For example it was incorporated into the coat of arms of the Kingdom of Naples. King Charles of Naples bought a claim to the title of king of Jerusalem in the 1270s, when the title was practically meaningless but still symbolically powerful. (The cross is not included in the current arms of Spain, but the king of Spain could, theoretically, use the title of king of Jerusalem, since the Spanish inherited it from Naples.)