This papercraft is a Teleporter, a major field of study at Black Mesa, and their portals were used primarily in the procurement of Xen crystals, as they were used to transport scientists to and from the Borderworld for research and analysis, the paper model was created by MastermanT.
The portal can be used to travel between Xen and Earth, however, portals that allow for travel within the same dimension can be found across the facility, notably in the Lambda Reactor Core, which contains 9 portals, three on each of its three levels. Both Gordon Freeman (through the Black Mesa's Lambda Core teleport) and Barney Calhoun traveled to Xen through such a teleport, to escape from Black Mesa before its destruction.
The Displacer Cannon, found and used by Adrian Shephard, can also allow for dimensional travels, although how a hand-held device is capable of teleporting itself and the user into Xen and back using internal power is a mystery.
During his brief visit to Xen, Barney is also required to activate and calibrate a "focal point", which was then used by Doctor Rosenberg to teleport several members of the science team outside of Black Mesa. [Source: wiki]