An interjection is a word or phrase standing alone or inserted into a sentence to exclaim or to command attention. Grammatically, it has no connection to the sentence. An interjection can be strong (Ouch! Ugh!) or mild (My, Oh). A strong interjection is followed by an exclamation point.
Ouch! That bee sting really hurts!
A weak interjection is followed by a comma.
Well, if you don't like it, leave.
An interjection inserted into a sentence may need a comma before and after it or no commas at all.
The time, indeed, has come.
What in the world did you mean by that?