Announcements are mere statements posted by the Admin or HR regarding anything happening in the company, reminders, occurrence etc. Not everyone can create an announcement, only certain employees are given the privilege to create them and typically HR or top management are given the rights.
When an Announcements occurs, typically every employee is sent an email notification about it and you can also view it directly on the platform. So next time when you get an announcement, look for this on your home page.
You can click 
Announcement icon in left hand navigation panel to view all the announcements.