To work efficiently with data, it is often necessary or helpful to sub-divide a data. Spaces allow you to do this with the help of tags. A Tag is just a text that can be used to only concern yourself with a subset of the data. They are completely up to you (or the owner of the Space you are working with).
So you might want to use different tags for the different ways you want to use or style data. For example, you might want to tag the Store location you are putting in a layer with the amenities that the particular store provides (coffee, food, late_night). In addition you could use new as a tag for newly opened stores you want to highlight and call out on the map.