budget constraint
Wanting things is free; buying them is not. The budget constraint is the simple, unglamorous fact that you cannot spend more than you have. It is the boundary that separates the bundles of goods you can afford from the ones you only wish you could. Every choice in consumer theory happens inside this fence.
Put as an equation, the budget constraint says total spending cannot exceed income, or for two goods, price of X times quantity of X, plus price of Y times quantity of Y, must be no more than your money M. For example, with 12 dollars, apples at 2 dollars and bananas at 1 dollar, you could afford 6 apples and 0 bananas, or 0 apples and 12 bananas, or many mixes in between, like 4 apples and 4 bananas which costs 12 exactly. The set of all affordable combinations is your budget set, and its outer edge, where you spend every cent, is the budget line.
The budget constraint is half of the whole story of consumer choice; preferences are the other half. It reminds us that economics is fundamentally about scarcity and trade-offs: to get more of one good, you must give up some of another. When your income rises, the constraint loosens and you can reach better bundles; when a price changes, the constraint tilts, and that tilt is what ultimately traces out a demand curve.
With a 15-dollar phone plan budget, you can pick more data and fewer texts, or more texts and less data, but not both maxed out. The plan menu is your budget set, and the trade-offs along its edge are the budget constraint at work.
The budget constraint marks what you can and cannot afford.
Income and prices set the constraint, not your wants. Raising the limit usually means more income or lower prices; merely wanting more never moves the fence outward.