I'm not native english speaker and can't give you a definition from Oxford about what tags are. But I think of them as color label with some text on it so can find, identify things or sometime just to make the kanban card view looks better. Just another quick and easy way to categorize things. Maybe it's similar to the use of #hashtag on social media (maybe not similar but exactly as hashtag is a tag with a hash). Lots of objects can have tags in Odoo, sometimes I add tag capability to those who doesn't have. Just do a search look for task_ids in Odoo python files and you'll find out which have tags.