What are "distributions" in automake?

In a Makefile.am file I saw the following entry:

EXTRA_DIST = bootstrap \
	ChangeLog \
	config/config.rpath \
	m4/ax_pthread.m4 \
	m4/iconv.m4 \
	MacOSX/someheader.h
    [...]

and I wanted to find out what it means, but the documentation doesn't seem to define what "distribution" actually means. It doesn't mean "compiled lib", does it? It's a source code distribution of some sort? (why would anyone need that? We can just use git or zip the root folder?) Hope someone can clarify.