# the Lord Yahweh says this

"this is what I, the Lord Yahweh, say"

# to them

"to my flock"

# Behold!

"Indeed!"

# I myself

The word "myself" emphasizes that it is Yahweh who will judge.

# will judge between the fat sheep and the thin ones

"will make sure that the fat sheep and goats and the skinny sheep and goats are treating each other fairly"

# the fat sheep and the thin ones

The leaders and strong people in Israel are spoken of as if they were fat and strong animals in the flock. The poor and weak people in Israel are spoken of as if they were the thin and weak animals in the flock.