Ambiguity at a semantic level in NLP is about the meaning of text, for example, "I saw her duck" might I saw her drop down suddenly, or that I saw her pet poultry. Note here how the zsemantic ambiguity related to lexical ambiguity of 'her' (as accusative or possessive pronoun) and 'duck' (avoidance action or bird).
Used on Chap. 13: page 290