This patch creates custom templates for units and structures used in Introductory Tutorial. The custom units can only build structures required by the tutorial, and the custom structures can also only trains units and research technologies required in the tutorial.
There are some exceptions, for example Storehouse can still research first wood lumbering tech and Temples can train Healer which are not required by the tutorial but could be some kind of sneak peek for curious new players.
The purpose for this patch is to ease new players going into Introductory Tutorial by preventing them to be overwhelmed by the game UI and possibly unintentionally hinder them from finishing/enjoying the tutorial.
Example of Forge that can only research tech with sword icon (the tech name in the instruction is incorrect, but that's for another patch)