The Uncategorized tile counts non-deleted transactions for your location that have no category at all. Its subtitle reads All caught up! when the count is zero, otherwise N transactions to categorize. Note that transactions imported from your bank are automatically placed in the Uncategorized Income or Uncategorized Expense account, which is a real category, so they are not included in this tile; to find them, look for bold (unreviewed) rows in the Transactions list instead.