What does “Isaiah said this because he saw Jesus’ glory and spoke about him” (John 12:41) mean?

This is a straightforward verse about the prophets looking forward to Jesus. The mention of Jesus’ glory may confuse some readers who are already confused about John 17:5 (see the separate answer on John 17:5 “glory that I had with you before the world existed.”). But John 12:41 is a straightforward verse showing what the Old Testament prophets did – foresee and foretell the future.

The full context is here:

³⁷ Even after Jesus had performed so many signs in their presence, they still would not believe in him. ³⁸ This was to fulfill the word of Isaiah the prophet:

“Lord, who has believed our message
and to whom has the arm of the Lord been revealed?” (Isaiah 53:1)

³⁹ For this reason they could not believe, because, as Isaiah says elsewhere:

⁴⁰ “He has blinded their eyes
and hardened their hearts,
so they can neither see with their eyes,
nor understand with their hearts,
nor turn—and I would heal them.” (Isaiah 6:10)

⁴¹ Isaiah said this because he saw Jesus’ glory and spoke about him. (John 12:37-41 NIV)

This is similar to what Peter said about the prophets:

10 Concerning this salvation, the prophets, who spoke of the grace that was to come to you, searched intently and with the greatest care, 11 trying to find out the time and circumstances to which the Spirit of Christ in them was pointing when he predicted the sufferings of the Messiah and the glories that would follow. (1 Peter 1:10-11, NIV)

So there is no difficulty with any of these verses. As Jesus said “Abraham saw my day” – the Old Testament prophets and patriarchs saw Christ in the future.