A portal is a technology used by the Combine and also somehow by John Freeman in order to transfer mass from one point to another presumably instantaneously. The mechanics of the portal are not well known, least of all in the "Full Life Consequences" version, as opposed to the Half Life 2 version. The references to the portals in the subject of this wiki are made twice, both at the same time. John Freeman emerges inexplicably from a portal, only to strike a macho pose, at the very end of "Hero Beggining." When this is described by the writer, it is stated that it was "a portal like the Combines" presumably referring to the portal storm they were attempting to create when destroying the tower, at least in the Half Life 2 version of events.
Specifics are few and far between.