This is an Accessibility feature. If you enable spoken text, the iPad will read any text you select. The text can be a mail message, a note, reminder, a web page - anything that can be selected.
To set it up, go to Settings, General, Accessibility. Turn ON - Speak Selection.
There are options to personalise your listening experience. Select Dialects to change to a different country. Eg Australian English, British English. Experiment with the Speaking Rate to get the speed how you like it. Move the slider to the right (the hare) to go faster and to the left (the tortoise) to go slower. If you want to follow along as the text is read, turn ON - Highlight Words.