The equation "X = 400" indicates that the variable X is equal to the value 400. This means that whenever X is referenced in a mathematical context, it represents the number 400. This type of equation is often used in algebra to define relationships between variables. In practical terms, if X represents a quantity, such as dollars or meters, then having "X = 400" means there are 400 of that quantity. For example, if X is the number of apples, then there are 400 apples. This simple equation helps in solving problems and understanding numerical relationships.