“Please note that these dates are tentative.”

You're in charge of planning a release schedule for the software that your team creates at work. You're sending the schedule for the next 6 months to a large group of people. You want everyone to know that the dates might change a little. In your email, you write:

Please note that these dates are tentative.

Please note that (clause)

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.

dates are tentative

"Tentative" dates are ones that might change.

Other things that can be "tentative" are:

tentative plans

a tentative answer

a tentative agreement

take tentative steps toward (something)