If you mistranslate something, you don't accurately describe or convey its meaning. It's easy to mistranslate a poem when you convert it from Spanish to English, failing to make its original meaning clear.
When someone literally mistranslates a piece of writing or a section of speech, they use the wrong words when translating from one language to another. You can also mistranslate the intention of someone's words, even if you're not actually translating between two languages: "I managed to mistranslate my grandmother's message when I gave it to the gardener." Translate means "turn from one language to another," and mis- means "badly."