The Common Core Standards for Mathematics call for students to obtain and demonstrate not only conceptual understanding and problem solving, but also procedural skill and fluency. Documents released by Student Achievement Partners go on to describe procedural skill and fluency as speed and accuracy in calculation that enable students to apply mental resources to more complex concepts and processes.
Standards in grades K-6 use the word fluent, or fluently, to explicitly call for students to achieve quickness and accuracy in calculations. In later grades the standards themselves do not use the word fluent or fluently. By middle school, fluency is less about calculation, and more about ease of manipulation of expressions, equations, notations, etc. Thus fluency exercises are applied toward any skill needed to manipulate expressions and equations with ease. For example, in Grade 8 students should work towards quickly and accurately solving general, one-variable linear equations.
An additional resource for guiding fluency work in later grades is the PARCC Model Content Frameworks for Mathematics.