Interjection is a word or a short exclamation that conveys emotion. It has no real grammatical value nor connection to any other part of the sentence.
The word "interjection" literally means "thrown in between" from the Latin terms "inter" ("between") and "iacere" ("throw").
The following familiar interjections in vocal English are: ah, eh, er, um, hey, oh, ouch, uh, huh, well. They are usually expressed in a spoken form rather than in writing.
Learn some of these common, basic ASL interjections that are often found in basic ASL classes and in everyday: #ok (ok or okay in English), oh, and pshaw.
One of the most common ASL interjections is #ok.
ASL interjection: oh, oh-i-see.
Non-manual grammar is also important and may be accompanied to convey a subtle meaning. For example, one uses a "oo" mouth bound with the ASL sign "OH". It's "oh". But, when the mouth morpheme is "ahh" with the sign "OH", this meaning is more of "Oh I see!"
Interjection as in "oh stop it", "oh please", "no big deal", "no, it's all right", ...
These interjections are a few examples. Social immersion or interaction is necessary to fully understand how to use them correctly in subtle various contexts.
As for more advanced learners, below are some more interjections.
The gloss for this ASL sign is intej-awful. It's not literally "awful". As an interjection, it means along the lines as "my goshness", "it's a terrible news", "it's unfortunate", "gosh, how horrible!", and such.