Z”L after a person’s name means that that person is dead. It is an abbreviation for the Hebrew phrase Zichrono livracha. [Of Blessed Memory.] The feminine form is zichronah livracha. The correct way to pronounce the abbreviation is “zahl.”
Jews love acronyms. If there is a phrase that takes a long time to write, why not just abbreviate it it? Added bonus: that way you don’t have to spell it! And if you put a vowel or two back in there, you can make it into an acronym!
When a word in Hebrew is abbreviated, there’s a little sign put into the letters that remain to clue you in to what’s going on. It looks like a lone quotation mark and most people refer to it as a “choopchik.”
Z”L = Zichrono + choopchick + Livracha = Of Blessed Memory, or “ZAHL”
in Hebrew, it looks like this:
And for your further edification and amusement, here’s a list of other common Hebrew and Hebrew-ish acronyms.