127 - Tags are keywords/terms assigned to a piece of information to facilitate later retrieval. There are primarily two different type of tags I think about, keyword tags and thematic tags.
Keyword Tags - you can use tags to create a unique index. You do this by giving each note a tag specific enough that you will only ever have a couple notes per tag. When using tags in this manner, you do not want to tag every note. Instead you just want to create entry point into all the notes you have on a topic. An example of this would be creating a #cognitive-skills tag that leads to this note, which is the start of a sequence of notes on cognitive skills.
Thematic Tags are how tagging is typically done. Often a note will be the intersection of multiple ideas, especially when you are remixing information. For example, my note sequence on the future of work would have both the tags #future and #work. Then when I go to search I type in multiple tags to find if I have notes on the intersection of different topics I write about. It can be especially useful with large note collections if you create a heat map out of them and see if any themes emerge that you didn’t know about.