Question:
I was told that there is supposed to be some Hebrew letters on a memorial grave marker/headstone. We are planning a headstone for my deceased mother; can you help me with this?
Answer:
I am sorry to hear about your loss. I offer you the traditional condolences given to mourners: "May G‑d console you, together with all mourners of Zion and Jerusalem."
Most likely, the letters you are referring to are .ת.נ.צ.ב.ה (or they may appear as תנצב"ה). It is customary, but not obligatory, to put these letters at the bottom of a monument. These letters are an acronym for the Hebrew words תהא נפשו/ה צרורה בצרור החיים (t'hay nafsho/ah tzrurah b'tzror hachaim), "May his/her soul be bound up in the bond of life." This paraphrases the words that Abigail told King David (I Samuel 25:29): "But my lord's soul shall be bound in the bond of life with the L-rd your G‑d."
Click here to find more information about putting up a monument. I also advise you to visit our Death and Mourning section; you are sure to find there much useful information and insights to help you get through this difficult period in your life.
All the best,
Rochel Chein for Chabad.org

צרור החיים is the last and highest level of Gan Eden (paradise) as according kabbalah there is 14 degrees in gan Eden, 7 for the lowest and 7 for the highest.
You have to have another jugement to pass through one to the other highest. And being already a righteous*tsadik* that have passed of course the first jugement to reach the first gan Eden.
No more jugement in צרור החיים that can be translated into eternal life. It is also the place where you are in the stage before the resurrection.