"Take it easy, it's one of a kind" - maybe you were carrying an object and you were running, and someone said this to you. It means "calm down / be careful, this object is unique / you won't find another one like it".

"One of a kind" basically means that the object is unique, there isn't another one anywhere in the world. However we don't always mean it literally - there may be others, but they are hard to find.