If we go deep enough, there is only pain and pleasure, satisfaction/dissatisfaction, happiness/unhappiness. All else that we sense - both what we call abstract and what we call concrete - are just guideposts telling us how to direct our actions to attain more satisfaction/pleasure. Some of these guideposts are unintelligible, and some of them are unreliable, but they are all just guideposts. Seeing some guideposts will bring automatic pain or pleasure, and our natural curiosity means we enjoy gathering additional, more reliable guideposts. But they are still just guideposts. All of them are just meaningless symbols until they are interpreted as meaningful guidelines for directing one’s actions (or if we develop an automatic pleasure/pain response to the symbols, like some people automatically feel fear or revulsion when they see the infinity symbol or the dollar sign, after repeated experience of seeing such symbols then later feeling bad).
At a sufficiently deep level of analysis, even the sensations of seeing/hearing/smelling/tasting/touching a rock (that is, everything that we can possibly call a “rock”) are meaningless unless we either find pleasure/displeasure directly in those rocky sensations or we interpret those sensations as guidance toward relieving felt unease. If someone claims that some thing “exists” but it isn’t clear how that notion pays rent in your anticipated experience of happiness/unhappiness based on your actions, you can usefully ask, “So what?”
All that said, it still makes some sense to talk about equations like 1+2=3 being meaningless abstractions, because they are like unfinished sentences unless one finds believing “3” to be pleasurable in itself - such as because it is your lucky number. For most people, though, 1+2=3 is a like an unfinished sentence, in that it provides no specific guidance for their actions. It doesn’t tell them anything about what courses of action will lead to pain and pleasure until they interpret it into something like, “I have 1 banana here and 2 bananas there, and since I get full on 3 bananas I won’t need to go get dinner tonight.” Other than that, such a mathematical equality only has potential value as a tool, like having a screwdriver in your pocket: you might be able to use it as a guidepost to guide your actions, and it is comforting (satisfying) to have more tools at your disposal.
To put it concisely, Meaning = Utility.
At the deepest level, no sensations/symbols/etc. have meaning to a person until they are connected to their anticipated pain or pleasure, satisfaction or dissatisfaction. What we call “abstractions” are those symbols that are relatively more roundaboutly connected to pain/pleasure, or not connected at all (if not at all, we might them “formalisms”). We call symbols/beliefs “concrete” when their guidance of our actions toward satisfaction is more obvious, visceral, or automatic.
If I find that every time I see a red light, I will get a jolt of pleasure if I flex my index finger, that red light (a sensation, a symbol) has meaning for me. If I hear someone say “Duck!” that symbol has meaning for me in that I believe I could avoid some pain by ducking.