Learn French: C'est un ordre important, répète-le. - It is an important command, repeat it.

In positive commands (the affirmative imperative), the direct object pronoun (like le, meaning "it") moves to the end of the verb and is attached with a hyphen. If it were a negative command ("Don't repeat it"), it would stay in front of the verb: ne le répète pas.

How do we know to use le for "it" instead of la?

The pronoun "it" refers back to the command, un ordre. Since ordre is a masculine noun, we must use the masculine direct object pronoun le.

Doesn't the tu form of this verb usually end in an "s" (répètes)?

Normally, yes! However, for regular -er verbs like répéter, you drop the final "s" in the tu command form. That is why it is répète-le and not répètes-le.

Why does the sentence use C'est instead of Il est to say "It is"?

You use c'est when it is followed by a determiner (like un) and a noun. Because we are saying "It is a command" (un ordre), we use c'est. If we were only using an adjective, like "It is important", we would use il est (il est important).