Use this phrase to point out something that you want people to pay attention to in an email. You continue by explaining what you want your readers to pay attention to:

Please note that I will be out of the office starting Friday, September 2.

Please note that all applications must be accompanied by two letters of recommendation.

This is a very formal phrase. You can use this in public messages that you send or post to a large group of people, especially if you don't personally know all of them.