Isocost and isoquant are concepts used in the theory of production, which deals with the relationship between inputs (e.g. labor, capital) and outputs (e.g. goods, services) in the production process.
An isocost line represents all the combinations of inputs that a firm can purchase for a given total cost. The slope of the isocost line represents the relative prices of the inputs.
An isoquant represents all the combinations of inputs that produce the same level of output. The slope of the isoquant represents the marginal rate of technical substitution, which measures the amount of one input that must be given up to produce one more unit of output while keeping the level of the other input constant.
Together, isocost and isoquant lines are used to analyze the cost-minimizing behavior of a firm and the production efficiency of different input combinations.