If something is unambiguous, there are no two ways to interpret it. If your girlfriend burns all your letters, texts you that she hates you, and moves a thousand miles away, the unambiguous message is that she’s finished with you.
If you're nervously reading the manual on how to trap the rattlesnake that's hiding under your bed, you'd hope the instructions would be unambiguous and tell you very clearly what to do. The word unambiguous gets its meaning from un-, meaning "not," combined with the Latin -ambi-, meaning "both ways," and -agere, meaning "to drive." So unambiguous is not driving in two directions — it's going full speed ahead, in just one direction, with only one meaning.