One Shot AF is the EOS DSLR auto-focus mode that is best suited for still subjects.
In One Shot AF mode you can focus on your subject with a half-press of the shutter button (or by pressing the AF-On button). Once focus has been attained, you will hear an audible ‘beep’, which gives you the signal to go ahead and capture the shot. You can also recompose your scene while holding down the button, knowing that your focus has been set. This is called AF Lock