And now, O Father, glorify me together with Yourself, with the glory which I had with You before the world was. -- John 17:5 (NKJV)

Jeremiah says something similar:

“Before I formed you in the womb I knew you; Before you were born I sanctified you; I ordained you a prophet to the nations.” -- Jeremiah 1:5 (NKJV)

So according to God Himself, a person already exists even when he is only part of God's plan and not physically created yet according to our timeline. This is possibly because God is not limited by our time. That means God can already "sanctify", "glorify", "give grace" and "choose" a person that was not created yet, but only part of God's plan.

Therefore do not be ashamed of the testimony of our Lord, nor of me His prisoner, but share with me in the sufferings for the gospel according to the power of God, who has saved us and called us with a holy calling, not according to our works, but according to His own purpose and grace which was given to us in Christ Jesus before time began, but has now been revealed by the appearing of our Savior Jesus Christ, who has abolished death and brought life and immortality to light through the gospel, to which I was appointed a preacher, an apostle, and a teacher of the Gentiles. -- 2 Timothy 1:8-11 (NKJV)

and

Blessed be the God and Father of our Lord Jesus Christ, who has blessed us with every spiritual blessing in the heavenly places in Christ, just as He chose us in Him before the foundation of the world, that we should be holy and without blame before Him in love, having predestined us to adoption as sons by Jesus Christ to Himself, according to the good pleasure of His will, to the praise of the glory of His grace, by which He made us accepted in the Beloved. -- Ephesians 1:3-6 (NKJV)

Therefore, John likely meant that Jesus was on God's mind and part of His plan before He created anything. This is also in alignment with the opening of the same book in John 1:1.