Reconciling your budget is the act of matching what you have budgeted to what you actually spent.
The warning appears when there are discrepancies between your budget projections and actuals, starting from the campaign start date up to (and including) yesterday.
If your Today’s Data toggle is reflecting actuals, then the alert will show discrepancies from the campaign start date up until (and including) today.
Read our blog post series on the importance of reconciling here.