What does it mean to "re-tank" toilet?
I would take it to mean that he either installed a new tank on the toilet bowl, or re-installed the existing tank.

This might be done:


If the tank was damaged in some way.
If the tank was leaking at the tank-bowl union.
To change the look of the toilet, without replacing the entire unit.




It could also mean that he installed a new flush, and/or fill valve assembly.

This might be done:


If the toilet was leaking.
If the toilet was running intermittently.
If the toilet was not flushing properly.



NOTE: There are various types of fill and flush valve assemblies, these are simply two of the more common types.