What does Matthew 17:23 mean?

In making these statements, Jesus is not speaking in parables or obtuse language. He is not attempting to confuse the disciples. He truly wants them to know what is coming, and He is intentionally being clear about what will soon happen to Him. He is providing for them both by managing their expectations for what comes next and by giving them facts to remember after His death and then His resurrection.

He has told them that He, the "Son of Man," is about to be delivered into the hands of men. Now He adds that those men will kill Him and then He will be raised on the third day (John 12:32–33). Jesus willingly made Himself available to be sacrificed for the sins of humanity on the cross. He did this with full knowledge of what would take place (Matthew 16:21; 17:22–23). He was never surprised by a moment of it. He was acting in full obedience and submission to the will of God the Father, who gave His only begotten Son as this sacrifice (John 3:16).

The disciples were upset to hear Jesus say this. Still, they did not fully understand what it meant (John 2:22).