It means you are invited for an event that givea u lots of unexpected enjoyments.

This is an English phrase that essentially means that something (usually an event or show) is going to be so great that you are privileged or fortunate to be in attendance. This phrase is often used to introduce acts/events and also used in normal conversation to describe an event. Its frequently used to build excitement and anticipation for events to come in the future. For example, "Ladies and gentleman, tonight you are in for a treat because we have a very special guest tonight..." "Your birthday party is going to be so amazing, you are really in for a treat!"

It kind of means that you'll be surprised about what is soon to come. Sometimes it will be used for something positive or sometimes negative. Example: "My birthday is tomorrow! I can't wait to see what I will get!" "I have a present for you. You're in for a treat, I think you will like it."