Something unusual is uncommon. It's odd, weird, or out of the ordinary in some way.
Unusual is the opposite of usual, thanks to the prefix un, meaning "not." Unusual things are noteworthy: you don't see them every day. A dog chasing a squirrel is usual. A squirrel chasing a dog would be unusual. Unusual can mean strange and bizarre, or just things that are rare. If someone is always absent, showing up would be unusual. Most surprising things are unusual.