Something that is "official" is approved by the authorities (the people who are in charge). "Official" things are often written down or recorded somewhere.

When is the official start date?

They tell me that I've got the job, but I don't know. They haven't sent me an official offer letter or anything.

We often compare the "official" version of something to how it actually is in reality:

My official title is "Executive Coordinator" but basically I'm a secretary.