Pressing the shutter-release button halfway initiates autofocus. Focus mode lets you choose whether focus locks once the camera has focused or if the camera continues to adjust focus while the shutter-release button is pressed halfway.

AF-S (Single-servo AF)
For stationary subjects. Focus locks when shutter-release button is pressed halfway.

AF-F (Full-time servo AF)
For moving subjects. Camera focuses continuously until shutter-release button is pressed. Focus locks when shutter-release button is pressed halfway.

MF (Manual focus)
Focus manually.