To be pleased means to be happy about something.
"You will be pleased to know" means that whoever is speaking knows the recipient will be glad to hear whatever is going to be said after "you will be pleased to know..." It is expected that the recipient will be glad to hear whatever the speaker is going to say after the phrase.
For example: I have been looking for a textbook, but I haven't been able to find it for weeks. My friend might one day tell me, "You will be pleased to know that I've found the book you were looking for!"
It can also be used to announce something of good news.
Ex: "You will be pleased to know that Tavis and I have decided to move in together."
The phrase can also be used sarcastically, but that's a lot harder to explain. I'll comment if I think of a way.