“Eat your spinach!” is in the imperative

what does it mean?

[“Eat your spinach!” is in the imperative]

There are many types of ‘grammatical moods’ in English (more at: en.wikipedia.org/wiki/Grammatical_mood ), but the imperative is the one that COMMANDS you to do something. Here are a few examples of the imperative form:

As you can see, the most important thing about the imperative is that it begins in a bare infinitive.
You can change between different moods by moving the sentence around a little…:

(The above are in the subjunctive form/mood - the one which expresses doubt about something).

Now, back to your sentence ‘“Eat your spinach!” is in the imperative’. This simply states that the sentence ‘Eat your spinach!’ is written in the imperative mood. I hope this helps you!