Truly I say to you

"I tell you the truth." This adds emphasis to what Jesus says next.

this generation will not pass away

Here "pass away" is a polite way of referring to death. AT: "the people living today will not all die" (See: rc://en/ta/man/translate/figs-euphemism)

"until God causes all these things to happen"

"Heaven and the earth will no longer exist"

Here "words" refers to what Jesus has said. AT: "what I say will always be true" (See: rc://en/ta/man/translate/figs-metonymy)