It's analogous to a combined count of how many people have commented on the post, or shared it (or "reblogged", to use the Tumblr term). It might also include likes? I only use Tumblr occasionally so I'm not sure.

I don't know why Tumblr counts things like this. I sort of like it though — it makes it feel like a distinct place. Tumblr hasn't escaped enshittification, but it makes me happy that it still exists as a little pocket of weirdos