The "Tips and notes" section for Intro 1 describes two kinds of uses for E, the Imperative E and the Vocative E. This kind is the imperative E because the speaker is telling someone to do something. "E" makes ʻai into a command-type grammatical format. On another page, someone was talking about how parts of speech are more fluid in Hawaiian, like ʻai can be noun (food) or a verb (eat). At least, that was my understanding of it! :D
Here's what the tips and notes say about these two uses of E:
E is used before an action to signify a command or a suggestion. When you say, "E hele", you're telling someone to "Go!"
E is used before a noun (usually a person) to indicate that the person is being addressed.
Ex. Mahalo, e Kawika. ➜ You are saying thanks to Kawika.