"May peace be upon him" or "Rest in peace" or other phrases to that effect. It is a customary thing to say when invoking the name of someone who has passed on. For example, I could say something like "My great-grandfather, alav ha-shalom, always used to say...". It is considered a respectful thing to say, and a way to properly respect those who have passed on. Literally Alav = on him (Ale-ha=on her), ha=the, shalom=peace. When discussing a great Rabbi or a very righteous person who is passed on, people often say 'Zikhron Tzadik Livracha' which means "May the memmory of the righteous be a blessing". Zikhron=memmory, Tzadik=a righteous person; "saint", Livracha=to/as a blessing. When someone was murdered especially with regards anti-Semitic massacres, people usually say "Hashem Yikom Domo" meaning "May God avenge his death". Hashem=respectful terminology for God, Yikom=will/shall avenge, Domo=his blood i.e. his death. There are other less well known phrases, but I believe this are the primary ones people hear/say.