understanding pgstatindex

Hey all.

When running pgstatindex(some_index) I'm getting the usual result:

Version, tree_level, index_size, root_block_no, internal_pages, leaf_pages,
empty_pages, deleted_pages, avg_leaf_density, leaf_fragmentation

with a bunch of numbers.

Now, my question are

What each one represent (aside from the label), what exactly is "average
leaf density" or "leaf fragmentation" or any other label?

And even more importantly, how can I make sense out of this number regarding
my index?

I've been told that if "Leaf Frag" is above magic number, then reindex. But
I'm looking for a more empirical approach.

Intuitively, I assume that an index state depends on many params that are
somehow reflected by those number, and if I'll understand those, I could
understand my indexes state better.

Thanks.

Shlomi.