The columns, in plain words
Most ageing reports show each customer on a row and the amount they owe in columns: current (not yet due), 1–30 days overdue, 31–60, 61–90 and over 90. Some reports age from invoice date rather than due date, so check which one you are reading. Ageing from the due date is more useful for collections; ageing from the invoice date is sometimes used for provisioning.
The total column should equal accounts receivable on the balance sheet. If it does not, there are unapplied receipts, credit notes not matched to invoices, or journal entries posted straight to the receivables account. Fix that first, or every conclusion you draw is unreliable.