What is the /initrd directory?

[Aaron Konstam <akonstam trinity edu>]

I am sorry if this is partially repetitive relative to a separately posted
question but it focuses more exactly on the problem we are facing.
We have machines with this directory and without. Some with this directory
have the directory empty.

Can someone clarify its use? Don't bother responding to both postings with
somewhat  overlapping questions.
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Aaron Konstam
Computer Science
Trinity University
715 Stadium Dr.
San Antonio, TX 78212-7200

telephone: (210)-999-7484
email:akonstam trinity edu