Semantic structure is a fancy term for an organization that represents meaning. For example, an English sentence is a semantic structure. Dog bites man.
Man bites dog.
Now, obviously the words have meaning. But, two sentences containing the same words in the same structure can have very different meanings - depending on where in the structure the words appear. So in the first case it is the dog who bites the man, and in the second case the man who bites the dog. To grossly oversimplify, in the case of our three-word sentences above, English grammar tells us that the word before the verb is the subject, and that the word after the verb is the object. Let's say I ask you to build a house out of blocks, and give you your choice of two different sets of blocks.