Learn Spanish (Latin America): Me lo dijo en voz baja.

It translates to "it." In English, we often just say "He told me" and leave "it" implied. In Spanish, you usually need the direct object pronoun lo to represent the actual thing, secret, or piece of information that was said: Me lo dijo literally means "He told it to me."

Why is the pronoun order Me lo and not Lo me?

When a sentence has two object pronouns, the indirect object (who receives the information, me) always goes before the direct object (the information itself, lo). A handy rule of thumb is that the "person" comes before the "thing."

Why use en voz baja instead of a single word like silenciosamente for "quietly"?

En voz baja (literally "in a low voice") is the most natural idiom in Spanish for speaking "quietly," "softly," or "in a whisper." When describing how someone speaks, Spanish strongly prefers using en voz baja or its opposite, en voz alta (loudly), rather than adverbs ending in -mente.

Since dijo is in the past tense, why isn't there an accent mark on the final o?