The symbol between date of birth and date of death is an Arabic invocation to God written in classical calligraphic style, and so is read from right-to-left. The Arabic invocation is "Ya-Baha-ul-Abha", which translates to English as "Oh thou the Glory of the Most Glorious!".
This invocation was revealed by the founder of the Baha'i Faith, Baha'u'llah, who called it "The Greatest Name" (of God). As such, it has special significance to members of the Baha'i Faith world-wide.
Note: in the early years of the Baha'i Faith, this invocation was commonly put on headstones, but this practice has largely ceased as it was later clarified by the leadership at the Baha'i World Center that it would be preferable to use the nine-pointed star, or the Baha'i ring symbol on headstones instead.