Learn French: Manges-en, c'est bon ! - Eat some of it, it's good!

You are exactly right! For regular -er verbs, we normally drop the "s" in the "tu" command form (like Mange ton repas !). However, when the command is immediately followed by the pronouns y or en, we add the "s" back for easier pronunciation. This avoids having two vowel sounds right next to each other.

Why is there a hyphen between Manges and en?

In affirmative commands (when telling someone to do something), object pronouns are attached to the end of the verb with a hyphen. If it were a negative command, the pronoun would go before the verb without a hyphen, as in N'en mange pas ! (Don't eat any!).

What exactly does en replace in this sentence?

The pronoun en replaces a noun introduced by a partitive article like du, de la, or des (meaning "some"). For example, if you were talking about cake, en replaces du gâteau. It translates to "some" or "some of it" here.

Why do we use c'est bon instead of il est bon?

When commenting on how food or a drink tastes in general, French speakers use c'est followed by the masculine singular adjective. Even if you are eating something feminine like an apple (une pomme), you still say C'est bon ! to mean "It tastes good!"