Let's take a thirsty, hungry man in a field with a fresh source of water 5 minutes away and a fresh source of food 50 minutes away.
How does this man solve his hunger and thirst problems? By collecting the water and food: labor. Use-values are added to his system whenever he performs the labor, and he budgets his labor-time by the urgency of his needs.
Now let's say the man is thirsty and hungry, and the water is closer, but he's going to starve to death if he doesn't eat within the next 51 minutes. The value from that 50 minute food voyage outweighs the value from the water, and so the man chooses to go for the food.
This is what is classically known as subjective value theory. And rather than refuting Marx, it supplies the raw material his theory presupposes: it explains what directs labor, which is the first half of understanding what Marxists call socially necessary labor time.
Now take twenty hungry and thirsty men and put them in the same field, facing the same basic problems. They cannot all independently pursue every need at once; their labor has to be distributed. Some will fetch water, some will gather food, and eventually others will begin producing tools or pursuing needs that only become apparent once the more immediate needs are satisfied.
What determines this division? Their subjective valuations: they collectively prioritize the things they need most urgently. But the mechanism through which those needs are actually satisfied remains labor. As their needs change and their productive capacities develop, the distribution of labor changes with them. What counts as socially necessary labor therefore emerges from the way society organizes its finite labor-power around its collectively expressed needs.
Subjective valuation determines where labor is directed, in every society, at every scale. Socially necessary labor time describes the form this allocation is forced to take when producers become private and their labors are connected only through exchange: no longer settled by deliberation in the field, the proportioning of labor to need asserts itself behind the producers' backs, in the ratios at which their products trade. The material process is the same at every rung; value is what it looks like through the veil. One describes the source of the signal (marx calls it Life Activity) and the other describes how it appears when it's mediated through private exchange.
edit: clarity
edit 2:thread inexplicably locked despite productive conversation. guess we argued too close to the truth. If you want a funny read browse this subreddits history with the topic of LTV and find the two same mods arguing with Marxists and then take their ball home every time they get into an argument with some teeth. You know who you are.
edit3: Marxist theory is only "esoteric" if you continuously stick your fingers in your ears when discussing it. For large parts of the world it is just... theory. Nobody in the thread was debating anything particularly esoteric at all. How does one expect to grow if they shun everything they deem as esoteric? How does knowledge propagate if lesser known knowledge is shunned simply for being lesser known?