The drainpipe on a kitchen sink is the piece of plumbing pipe that comes down through the drain hole and attaches to the P-trap under the sink. Most of the drain pipe is under the counter and out of sight. The portion you do see, the drain strainer, is the top of the drain pipe. If you detach the drain pipe from the P-trap to replace either piece, clear a clog or perform any other plumbing work, you can quickly reattach it.
Use the adjustable pliers to loosen the nut (turning counterclockwise) that holds the P-trap to the waste line, but do not remove it. Swing the P-trap into place directly underneath the drain pipe. Move the trap slowly because it may be full of water.
Retighten the nut holding the P-trap to the waste pipe, turning clockwise.
Push the threaded trap adaptor up over the end of the P-trap and onto the threaded end of the drain pipe. Tighten the adaptor by hand until it will no longer turn.
Wrap the adaptor in an old rag so you do not crack it or damage the finish, then tighten it as far as you can with the pliers.