newbie: what does "sign the message digest" mean?

[Ravi Malghan <rmalghan () yahoo com>]

Hi: I am very new to cryptography. I am reading a book
and donot seem to understand the meaning of "sign the
message digest" even after reading the chapter several
times. Below is what the book describes

A sender wants to send a message called "Message"
securly

1. sender computes the message digest for "Message".
2. sender signs the message digest and attaches the
resulting digital signature plus the certificate to
the message. The result is Signed Message + Sender
Certificate + Signature
3. sender then encrypts the result from step 2 with a
random session key
.
.
and so on

What does the Step 2 mean. I understand what is
computing a message digest. 
1. But don't understand what is "signs the message
digest". 
2. How is "Signed Message" different from "Message"
3. What is a Signature?
Can someone explain?

Thanks
Ravi

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