With a reconciliation you can ensure data integrity or consistency. A simple example is a comparison of two lists. There are many possible approaches to show which elements of list A are not in list B and vice versa.
In principle you can come up either with a realtime comparison or with a batch process to do this. The realtime reconciliation can be done with a worksheet formula (including a conditional format) or with a VBA function. As a rule of thumb you do not want its runtime to be unbearably long - let's say longer than 0.2 sec. The batch process can be a VBA subroutine or a pivot table. This might be your preferred choice if the reconciliation is quite complex or if it lasts quite long (minutes).
Here is an example for a conditional format (volatile worksheet formula):