The titular quote is something newcomers to [Expect] say ''very'' frequently.
It indicates a misconception or two.

This explanation deserves its own Wiki page:  
Expect is organized in terms of a dialogue.  It does not have a direct notion
of "the content of the previous send"; instead, we all have the expectation
that, after a command is sent, what appears before the next prompt is the result
of that command.  That's a convention, though.  From Expect's perspective, it's
all just characters going back and forth, with no privileged concept of "command",
"prompt", or so on.

Let's do a simple example:  what do you expect from 

    set prompt {$ }

    spawn ssh $user@$host
    expect "Password: "
    send $password\r
    expect $prompt
    send ls\r
    puts "The output is '$expect_out(buffer)'."

?  You'll probably see something like the logon message.  

To achieve what people think they want, it's necessary to write instead

    spawn ssh $user@$host
    expect "Password: "
    send $password\r
    expect $prompt
    send ls\r
    expect $prompt
    puts "The output is '$expect_out(buffer)'."

Do you see the difference?
----
It's sometimes useful to

    expect *            ; # Receive *anything* returned immediately.

and/or 

    expect timeout      ; # Receive everything returned up to $timeout seconds.

when one "just wants the answer".

Also, note that '''$expect_out(buffer)''' doesn't really hold what people want; 
it typically needs to be filtered down at least to eliminate the prompt.