All about bookmarking things that are interesting or important to you. One thing that seems pervasive when learning about all of these services is that you had better be prepared to "put yourself out there". If you don't want people worldwide to know so much about you then you won't want to be putting your information in places like this. However, if you want to find people and sites that share similar interests, then this is exactly how you would go about it. One of their guidelines is that you should bookmark or tag every site that you find interesting or relevant to what you are looking for. The more you do this, the more everything seems to come together and almost focus it's attention on you. The same can be said for Technorati and some of the other similar services.
Originally when designing web pages, you would have to put tags or "metatags" embedded in the sites HTML code so that search engines could "crawl" the world wide web and find your site and then catalog it for others to find more easily. This is just an evolution of that same process that gives non-technical people an easier way to do the same thing on their own.