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.