It's is the usual spoken form of 'it is'.
It's the best news I've heard in a long time.
这是我很长时间以来听到的最好消息。
It's is the usual spoken form of 'it has', especially when 'has' is an auxiliary verb.
It's been such a long time since I played.
自从我上次演奏以来已经有很长时间了。