Magnetraps are devices used by the Daleks to pull spacecraft down to a planet by magnetic attraction exerted by the spotlight-like devices on the four corners of the landing pad. When used, a ship can be guided down so gently that a sleeping crew won't awaken before the ship is boarded.
On Skaro, the Daleks used a magnetrap in an attempt to capture 2K, but he used a drill in his head to escape underground. (TV21: Impasse)
The Daleks later used a magnetrap to capture Kirid's spacecraft. (TV21: The Emissaries of Jevo)
On Gurnian, the Daleks used a magnetrap to capture the craft transporting Mary Stone and Andy Stone. (DWA: Monsters of Gurnian)
Behind the Scenes
While referred to in-story as a magnetrap, the feature Dalek War Machines in which it first appears refers to it as a magnatrap.