Here's what it's not: professional article writing. If you want to go through multiple rounds of editing, please do. If you want to write two thousand word epics about your topic of choice, please do. But it's also okay to write up a hundred quick words and post them without thinking twice about it.
When you blog, you're building up a body of work that represents you online. It's a gateway into your thought process more than anything else. So do what moves you - whether that's short thoughts, bookmarks you like, essays, fiction, poetry, photo albums, and so on. You do you. The only thing that's really important is that you keep doing it.
Thanks to Jan-Lukas Else for the pointer!