Follow the blogs you've been hearing about.
Share the things that you love.
That is literally the biggest piece of text on their main page. I just looked it up right now. If you can't figure out what Tumblr is about, that is a deficiency with yourself, not the platform.
It sounds to me like marketing bullshit or a slogan - it doesn't really describe anything.
What actually happens on the site?
Does it just display feeds of popular blogs, and you get to recommend some?