Paste-in-hole, or through-hole reflow, refers to the mounting process whereby a solder alloy preform (paste) is applied to the printed circuit board (PCB) vias (plated through-holes) prior to mounting components.  The  components are inserted into paste-covered holes and cured in an infrared (reflow) oven creating a permanent bond/joint.

Paste applied to PCB through-holes

After mounting and reflow