A large portion of relevant modules/templates have now been switched to cargo. Various usages of SMW throughout the wiki need to be replaced by the new functions, in particular item tables. If some pages do not show up but contain no errors, please null-edit them. To see how you can help with the port check out Path_of_Exile_Wiki:To-do_list/SMW_migration (and leave a comment on the talk page if you have questions).
Tags are internally used by the game to describe certain properties/aspects of entities. Those are then usually used later to govern certain rules about those entities, such as which modifiers can spawn which items.
Most entities, like monsters, areas, mods and items come with their own list of tags. The tags are used to describe some properties of those entities, for example currency items are usually tagged with