The Worker Drill (depending on the race of the player doing this it may be called a Drone drill, Probe drill or an SCV drill) is a technique used to break the entrance of an opponent's base that's being blocked by units.
The main concept behind this technique is that workers, when ordered to mine minerals, will no longer collide with other units and pass though them instead. When they are given a different command however, they will occupy their normal space again, moving any units they may have been passing through.
To execute a worker drill against an opponent who has units blocking the entrance to his base, a player needs vision of the opponent's minerals. A group of workers is then selected and ordered to mine those minerals. When they are passing over the units stationed in the enemy entrance, a move, stop or attack command is given to force the enemy units to move out of position and allow the player's own units to move in.
A Worker Drill may also be used on a player's own ramp if it has been blocked by an enemy Protoss player using Stasis Field to allow the player's units to move in or out of the base again.