Here "pass away" is a polite way of saying "die." Alternate translation: "this generation will not all die" (See: [[rc://en/ta/man/translate/figs-euphemism]])
Possible interpretations are 1) "all people alive today," referring to the people alive when Jesus was speaking, or 2) "all people alive when these things I have just told you about happen." Try to translate so that both interpretations are possible.