This means 'more than one possible meaning'. In most texts, ambiguity is a problem, but in some kinds of text, such as poetry or song lyrics, ambiguity creates an effective richness of meaning.
Ambiguity can stem from interpreting words or grammatical structures differently.
Lexical ambiguity: "He hid the money in the boot" the Wellington boot, or the car boot?
Grammatical ambiguity: 'The sniper shot the general in the church' Who was in the church - the sniper or the general?
Ambiguity is typically resolved, allowing us to choose the right meaning, through context or pragmatics - so If you were asked to call someone a taxi, you wouldn't be likely to reply with "you're a taxi" .