Omiyage is the Japanese word for 'souvenir'. It can be used for any kind of souvenir, but in the context of your school or workplace, it generally means those small mysterious snacks occasionally found on your desk when you arrive at work in the morning.
It's not expected for you to give out omiyage at all, but if you do go on a trip and manage to bring back some treats, it's a great way to start a conversation about your trip, talk to someone haven't had the chance to yet, or just generally make everybody think better of you.
When giving out omiyage, don't forget the people in the admin office and maintanence rooms. They will love you for it!
Acronym for Official Teacher of English.