As stated above, the Marriage Tribunal doesn’t dissolve marriages.
For a marriage to be valid, certain conditions have to be in place. If the couple, later on down the road, discovers that one of these conditions might not have been in place, and thus the marriage was never valid.
If they think they have a case for the fact that their marriage wasn’t valid, they can present it to the Church (Marriage Tribunal), and it determines if the initial conditions weren’t or were present. If they find that there were certain conditions that weren’t present at the time of the marriage, they conclude that the marriage wasn’t valid and never happened before.
So they don’t dissolve anything, they just recognize the fact that the marriage never happened in the first place.
Hope this helps!