Reserved pasta water is the water the pasta was cooked in. It contains some of the starch from the pasta, which makes it work as an emulsifier to allow the pasta sauce to cling to the pasta noodles. You can use a coffee mug to scoop out some of the water before draining the cooked pasta in a colander. Then, if needed, add a little bit at a time as you toss the cooked pasta with the sauce. And if you don't end up needing it, you can discard it by pouring it down the drain.