What does this mean?

It means exactly what it says: that these forums also known as the "Apple Support Community" will be unavailable at those times tomorrow.

Nothing to worry about, it's just a routine update / maintenance.

They should be back online after 10PM Pacific Time.

It means these user-to-user forums will be undergoing maintenance at that time. It will not affect any other part of your Apple online "experience."

If you look, it is also at the top of this page.