Tumblr is short-form blogging platform, inspired by tumbleblogs, which were a thing back in the day (another example of Big Tech lifting an idea and then walling it off). So it's like micro-blogging but without the word limit, so you can throw out a thought or a picture or a video, or go into greater detail if the mood took you. On Tumblr people would tend to have a few on different topics.

I much preferred it to Twitter as it felt a lot more free-form but usually focused on a topic.