the meaning of just a heads up?

English as a Second Language English Teacher Explanations (ESL Tests)
1
  1. Just a heads up—this is a new meeting, so you’ll need to upload content or attach the files that you want to include.
  2. Who can look at content on their own?
  3. This is also good for other presenters who might want to skip ahead to a slide they’re about to present

Please explain to me the meaning of the bold phrases in the above sentences. They appear in an Office Application Manual.

Many thanks!!!

2

  1. I want to warn you that this is a new meeting.

  2. seems strange. It is ‘on their own’ (who can look at it by themselves) or ‘on its own’ (in isolation).

  3. might want to miss a number of slides out of the presentation and go straight to the one they are about to present.

3

Thanks, mam!

Another question:
This evergreen calendar can easily be personalized by replacing the photos with your favorite family photos.

What does the adjective “evergreen” mean in this context?

4

Presumably, you can use it year after year, but I have never heard the phrase so can’t be sure.