Keywords are an engine feature introduced in Skyrim and continued in Fallout 4 and Fallout 76. Keywords are metadata that can be applied to entities in the game and then referenced by other entities and scripts. Essentially, they can be best thought as categories: A keyword places an entity in a category that can then be freely references by the developers.
Example[]
At Camp McClintock, military training can only be started by wearing a military uniform. This is done by checking whether the outfit and helmet worn by the player has the EN05_MilitaryFatigueKeyword and EN05_MilitaryHelmetKeyword keywords.