Hover over a mark and Tableau shows a tooltip: the fields that made the mark and their values. Two things decide what it says.
The Tooltip button on the Marks card holds extra fields. Drop a measure there and it appears in the tooltip without changing the view. Be careful with dimensions: a dimension on Tooltip is still part of the level of detail, so it splits the marks. To show a count of orders without splitting the bars, turn Order ID into a measure with Measure > Count (Distinct).
Click the Tooltip button itself (or Worksheet > Tooltip...) to open the tooltip editor. Fields appear as tokens in angle brackets, like <Region> and <SUM(Sales)>, and Tableau fills them in for each mark. Use Insert to add a token, and write the sentence around it.