Wide apertures, photographer close to subject, background proximity to subject (subject is far from the background) and focal length (longer focal length shortens Depth of field).
Put it simply, when you are using large apertures like F1.2 or F1.8 etc, only what you focus on and what is on the same plane as what you focused will be sharp while everything else behind or in front of it, will be thrown out of focus (bokeh).
Thanks guys!