A pronouncement is an official announcement, like the kind politicians make. It’s formal and comes from someone in charge. The mayor of your town might make a pronouncement about the importance of not littering.
When you make a pronouncement, you're certain about what you're saying and say it with authority. A school principal's pronouncement about not running in the halls might be broadcast over the intercom so everyone can hear it. A big brother might make a pronouncement about what’s for dinner. Feel free to ignore him, but not the principal or the mayor, unless you want to get in trouble. Pronouncement comes from the Latin word pronuntiare, "to proclaim or announce."