A tax reserve is cash deliberately set aside to cover tax you already owe but have not yet paid. Tax preparedness is how much of the estimated liability that reserve currently covers.
Tax is the most common reason a business with a healthy-looking cash balance is not actually in good shape. The money is in the account, so it reads as available, but part of it was never yours to spend.
Treating the reserve as separate is what stops a quarterly payment turning into a cash emergency that was entirely predictable months earlier.