There are a number of software notices to display message(s) to broad sets of Wikipedians.
CentralNotice
Shows to users across multiple Wikimedia projects. Can be targeted by language, country, and other options. Dismissable and shows only to users with JavaScript support. Logged-in users can opt-out of certain types of banners in the Banners section of Special:Preferences. Controlled at meta:Special:CentralNotice. See also meta:CentralNotice.
Sitenotice
Shows to all users; appears above the page title. Dismissable and shows only to users with JavaScript support. Controlled by MediaWiki:Sitenotice and MediaWiki:Sitenotice id.
Anonnotice
Shows to all anonymous users; if blank, the sitenotice shows to anons as well. Appears above the page title. Controlled by MediaWiki:Anonnotice. Not dismissable.
A watchlist notice that only displays to users in a certain geographical area, although geolocation is not perfect. Added and controlled by MediaWiki:Gadget-geonotice.js. See Wikipedia:Geonotice. Dismissable by JavaScript.