This is sometimes referred to as The Problem of Universals, which is essentially The Problem of Concepts.
Ayn Rand, in the Introduction to Objectivist Epistemology points out that the brain automatically integrates the sensations from our senses into percepts.
When you look at an apple on a tree, you see an apple on the tree. As you walk up to it, or around it, the brain continues to 'update' what you are seeing. When you reach up to grab the apple you feel its firmness, the skins texture, and even its temperature in your hand. When you take a bite out of it, and discover the flavor, the texture of the skin differing from that of the meat, and if you breath in through the nose to experience the aroma, the brain integrates or more loosely 'associates' all these different aspects together. What you see, feel, taste, smell and even hear the snap of the stem from the branch and the crunch as you bite into it is the percept of that particular apple.
By the time you've learned the word apple, you've probably seen and/or had many different varieties, and noted that they are similar in many respects (similarity is given to us perceptually), and differ from oranges, bananas, tomatoes, trees, rocks, stars and everything else that an apple differs from.
Without getting too technical here, the similarities that we see, taste, feel etc., let us know that apples can vary in size, color, taste, etc., but that there is something similar that we can integrate from all the various percepts into a single new unit called a 'concept'.
Ayn Rand points out that the process is completed when we give this concept a name, in this case the name or word 'apple'.
When we do the same thing with sugar, candy, apples, oranges, grapes, bananas we can isolate a particular sensation of 'sweet' a sensation of taste that is similar, differing from the taste of say a jalapeño pepper, dill pickle, salt, a twig of a tree, dirt, etc.
When we form the proposition "Apples are sweet", we are stating that entities in the world that are perceived as apples possess the quality of sweetness that is in common with the percepts we abstracted the concept of sweet from.
When I think of an apple, I don't really have one static 2-d image that comes to mind. Instead, it's more like a 'flood' of apples, red, dotted or striped with say yellow, sometimes with a stem but not necessary, the little dried parts at the bottom where the flower was along with other memories of previous encounters with various specific apples. These are all associated together and filed away in the brain analogous to a manila envelope with the label "Apple" on it.
When I hear or read the statement "Apples are sweet"(1), I usually think - "Yes, they certainly are, this is true."(1) with true being the quality of the proposition aligning with and meeting all the criteria of the factors and processes that made the proposition possible.
(1) This is a simplification. Many apples are sweet. That some apples are tart, or even blah when stored to long, etc., are variations beyond the scope of this presentation. Apple was selected since it was used in the question being addressed.
