Anything that is based in semantics is to do with meaning and implication, whereas structures in language are identifiable arrangements of words or parts of words from which a certain meaning is inferred. As such, semantic negation is when a sentence is made negative through the implication of the words' meanings themselves, rather than because of specific structures or formations associated with negation.
An easy way to think about it is to imagine having learned English as a second language. The simple marker of structural negation in English is the addition of not. So, those who have learned English as a second language will easily be able to identify structurally negative sentences by the inclusion of not in some form: Andrew did not want to go or He wasn't hungry.
Semantically negative sentences would be more difficult for these English-learners to identify on first glance, because the words that inform the negation have the negation contained in their meaning. As in, He disavowed ownership or, uttered sarcastically, Well, that was nice of him! An English speaker will be able to interpret straightforward semantic negation according to the meaning of more complex words, or sarcasm from tone of voice, but these sentences are not as easy to identify as structural negation using forms of "not."
Sentences in English may be negated (made negative) by negation markers or by sentence meaning. Negative markers are no, not, neither, none, nor, etc. Negative sentence meaning conveys negation through a variety of different means, for example, by using negating words such as decline, absent, renege, revoke, refuse, etc.
These two methods of sentence negation are called structural negation and semantic negation. Structural negation is when a sentence is negated (made negative) through negation markers: Al did not wish to see the film. The negative marker not is placed after the modal verb in the sentence of negation. These sentences of negation use a modal verb that precedes the negative marker: did not.
Semantic negation is when a sentence is negated through sentence meaning: Beatrice declined to be civil and amiable to Benedic. This can be paraphrased as Beatrice was not civil nor amiable to Benedic. This method of negation may be harder to understand because the negation is contained in the vocabulary rather than in easily recognized markers.
Another means of semantic negation is through ironic sentences that mean the opposite of what they directly say, for example, That will teach you to go on blind dates as paraphrased actually means, That will teach you NOT to go on blind dates.