"So they will be called oaks of righteousness, the planting of the Lord that he may be glorified."
Isaiah 61:3b

The root definition of robust is "oak, strength". Robust means physically strong, or marked by richness or fullness of flavor. It can also refer to ethereal strength in the sense of an argument or faith, or the strength of a signal. A team could have a robust defense. A cruise ship could have a robust structure that can withstand sinking.

If we think of the word picture of oaks as meaning strength, how does this help us interpret Isaiah 61:3? If we back up and look at the context, we discover that this is the passage that Jesus read from, but you will note that Jesus stopped after the first half of verse two. He was proclaiming the year of God's favor and not vengeance. However, Isaiah writes here about what the ministry of the Spirit of God does to people:
to bring good news to the poor;
he has sent me to bind up the brokenhearted,

to proclaim liberty to the captives,
and the opening of the prison to those who are bound;

to proclaim the year of the LORD's favor,
and the day of vengeance of our God;

to comfort all who mourn;

to grant to those who mourn in Zion--

to give them a beautiful headdress instead of ashes,
the oil of gladness instead of mourning,
the garment of praise instead of a faint spirit;
that they may be called oaks of righteousness,
the planting of the LORD, that he may be glorified.

The last to is the one I am concerned with right now, but it's important to know that it is coupled with the others. Many times, I have heard people pray the first part of verse three over people during ministry times: "give them beauty for ashes". I did not know the context, but it sounded pretty good, and "the oil of joy instead of mourning and a garment of praise for the spirit of heaviness"(KJV). They seemed to always stop there, but Isaiah goes on to say that these healed people will be called "oaks of righteousness".

We got the meaning of oaks, but what does righteousness mean. What would it mean if you were called or described as a robust person of righteousness? If were peruse Isaiah 61 a few more verses down to verse ten, he shows us the meaning of righteousness:
I will greatly rejoice in the LORD, my soul shall be joyful in my God; for he hath clothed me with the garments of salvation, he hath covered me with the robe of righteousness, as a bridegroom decketh himself with ornaments, and as a bride adorneth herself with her jewels.
God grants salvation and deliverance and makes you righteous. He covers you and puts you in right standing, sets you in the right place. Then as this covered person, you can live a righteous life. Not perfect, but when God puts you in right standing with Himself, you can't help but do right in your life. Without the later, the former isn't genuine; but the later never creates the former.

So, ministry by the Spirit of God: the proclamation of the good news to the poor, the binding up of the brokenhearted, the proclamation of liberty to the captives, freeing the bound from prisons, and proclaiming God's favor; creates robust people walking in God's righteousness.

Making people into figurative oaks is God's work, "the planting of the Lord", and God does this to bring glory to himself, "that he may be glorified". Remember the Westminister confession catechism says, "“What is the chief aim of man?” “To glorify God and enjoy him forever.”

Robust. Like an oak. Planted by God, glorifying Him.