I believe what he means is that they build their first two supply depots and their first barracks right at their ramp, so that enemy troops (zerglings and such) cannot just run in and attack the mineral line.
As far as if it is possible to make an AI do it, its probably possible, but as of yet there is no way to make the terran successfully wall-in (to my knowledge). Most AIs use one of the prewritten blizz functions, AISetStock or AIBuild etc, and these functions do not have a parameter to decide where the building should be placed. In order to make the computer wall-in, a pathfinding function would have to be written, and a custom build script would have to be made, that can locate the points to build the depot and barracks. Either that, or the script would be specifically programed for each map.
