In consumer theory, the budget set is the collection of all possible consumption bundles that a consumer can purchase with a fixed income at given prices of goods. The equation defining this relationship is the budget constraint: p1x1 + p2x2 ≤ M, where p1 and p2 are the prices of goods 1 and 2, respectively, and M is the total income. Graphically, the budget line is a straight line representing all combinations that exhaust the consumer's entire income (p1x1 + p2x2 = M). Shifts in the budget line can occur due to changes in income or prices, affecting the affordable combinations of goods and thereby influencing consumer choices.