Why do individuals reconstruct that a waitress brought the menu?
Individuals need not always “create” a coherent knowledge structure. In many cases, they can rely on their prior knowledge to provide them with an already existing structure. Not surprisingly, this existing structure has a pronounced impact on storage and retrieval processes. If the incoming information is consistent with the prior knowledge, it is sufficient to store a link to the prior knowledge structure rather than to store the new information again. For example, when one of the party guests talks about having gone out for dinner, the listener does not need to store that the guest made reservations, that the waitress escorted them to the table, and that she brought the menu, etc. All this information is already part of the person’s general knowledge about “going out for dinner.” When retrieving the information at some later time, individuals simply need to recall “going out for dinner,” and then can reconstruct that a waitress brought the menu.

Question

Why do individuals reconstruct that a waitress brought the menu?
Individuals need not always “create” a coherent knowledge structure. In many cases, they can rely on their prior knowledge to provide them with an already existing structure. Not surprisingly, this existing structure has a pronounced impact on storage and retrieval processes. If the incoming information is consistent with the prior knowledge, it is sufficient to store a link to the prior knowledge structure rather than to store the new information again. For example, when one of the party guests talks about having gone out for dinner, the listener does not need to store that the guest made reservations, that the waitress escorted them to the table, and that she brought the menu, etc. All this information is already part of the person’s general knowledge about “going out for dinner.”  When retrieving the information at some later time, individuals simply need to recall “going out for dinner,” and then can reconstruct that a waitress brought the menu.

Answer

What it means is that when someone hears the phrase "going out for dinner" (or more accurately "went out for dinner"), they just assume, based on existing knowledge & past experience, that this included a waitress (or waiter) bringing the menu.
It just means that when someone thinks about going out for dinner, it includes other smaller components.
Is that what you were asking?

Answer

Notice how the author used RE-construct? That means that the information is already in the person's head and all they need to do is to remember / recall.

Once you remember the name of the restaurant that you visited, your brain remembers the sign, the design furniture, seats maybe even how the waitress looks like. You don't need to create a memory, you simply recall and your brain will reconstruct the scenario.