Na+/K+-ATPase is an enzyme found in the plasma membrane. It is sodium-potassium adenosine triphosphatase, also known as the 'Na+/K+ pump', 'sodium-potassium pump', or simply 'sodium pump', for short. It was first discovered by Jens Christian Skou who won a Nobel Prize in Chemistry in 1997.
The enzyme moves Na+ (sodium) ions out of the cell and replaces them with K+ (potassium) ions. This keeps the Na+ ions outside of the cell membrane, and keeps the K+ ions on the inside of the cell membrane. The process works in the opposite direction of diffusion.
For every three sodium ions that get pumped out of the cell, two potassium ions get pumped into the cell. One phosphate group is still bound to the pump.