Mathematically proficient students make sense of quantities and their relationships in problem situations. They bring two complementary abilities to bear on problems involving quantitative relationships: the ability to decontextualize—to abstract a given situation and represent it symbolically and manipulate the representing symbols as if they have a life of their own, without necessarily attending to their referents—and the ability to contextualize, to pause as needed during the manipulation process in order to probe into the referents for the symbols involved. Quantitative reasoning entails habits of creating a coherent representation of the problem at hand; considering the units involved; attending to the meaning of quantities, not just how to compute them; and knowing and flexibly using different properties of operations and objects.
The information below comes directly from the Common Core State Standards. For each practice we have quoted the Standards document so you have the exact wording at your fingertips and included a link for you to go to the Common Core webpage from here to see this information in context. You can also follow the links on the bottom of the page to see where on the site we've highlighted this particular practice.