The Imperative mood of a verb is used to give directions, or orders, or advice, etc, which is of course what is being done in this sentence.
In its POSITIVE form, it’s relatively straightforward. For mangiare (to eat) for example it’s simply:
- (tu) mangia
- (Lei) mangi
- (noi) mangiamo
- (voi) mangiate
- (Loro) mangino
Though in reality you would normally only ever use the tu, Lei or voi forms.
Note also the way that in -are verbs the last letters for tu and lei are generally the other way around from the good old fashioned present indicative form that we all know and love.
So I might say to my friend:
Mangia qualcosa, Giorgio.
(Eat something, Giorgio.)
Unfortunately a NEGATIVE instruction has a little quirk that you just have to get used to; you use the infinitive form as you saw in that question. So if I wanted to say “Do not eat, Giorgio”, it would be:
Non mangiare, Giorgio,
just as this question has “Non spaventare”, “do not scare”… etc.
Just to make that quirk worse… it relates specifically to the “tu” form, so you can assume that the person is speaking to someone informally. For “voi” or “Lei” it just uses the “non whatever the standard verb form is” style.
Thus if you were speaking to someone formally, it would actually be “non spaventi il bambino”.
The imperatives of spaventare being:
- tu spaventa
- Lei spaventi
- noi spaventiamo
- voi spaventate
- Loro spaventino
I was hoping to find a definitive reference for this (or something better than “because Vorenus said so”, anyway) and while I came up with several there are none that I would regard as definitive. However the ThoughtCo articles generally explain some issues well enough; take a look at this page:
and scroll down to the heading “How to Form the Negative in the Imperative”. It probably doesn’t add much to what I’ve already said but at least it’s an additional confirmation.
Unfortunately it’s one of those odd little rules that you just have to get used to, though it may take some time since most students rarely need to use the imperative in daily conversation, and the negative imperative even less.