That means
"I spoke this message to my secretary or into my computer, and my secretary or my computer did the writing. I didn't read what they wrote before it got sent to you. So if you find any mis-spellings, or funny ridiculous words that obviously don't belong here, it's all their fault, not mine. I was not only too lazy or busy to make the effort to write to you myself, I was also too lazy or busy or didn't care enough to check it over before it went out to you."
There's a bit of my own opinion in there, but I'm sure you get the idea.
"Dictated but not read" means that a message was spoken and transcribed by someone else without the original speaker reviewing it. This indicates that any errors in the message should not be attributed to the speaker. It emphasizes the importance of reviewing written communication to avoid misunderstandings.
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