The equation "f(b) = 5" indicates that when the function f is applied to the input b, the output is 5. This means that for the specific value of b, the function produces a constant result of 5, regardless of other values of b. In this context, f could represent any mathematical function, such as a linear function, quadratic function, or any other type. The notation shows a relationship between the input and output, helping to understand how functions operate in mathematics.