It's a list of lots of things I've bookmarked. I've been keeping this for almost 15 years now. Over the years, I've used different services as an easy way to track interesting things I've found on the internet. It started with Digg and del.icio.us (if you remember that) but as systems died I found it useful to own the data. For awhile I used Tumblr and that name stuck. Today it's fed by a Google spreadsheet and updated via email. Here's some more detail, if you're interested.
One of the things I really like about this is that I can go back over time and see a history of the things I was reading or thinking about. It's easy to go back and quickly lookup things from a week or a month ago that would otherwise be lost to me. A Tumble a simple thing, but it's gotten a lot of use.