NOTE: The assumption here is that you already have Gmail Label Share chrome extension installed and you have a shared label

What is Smart Label?

The Smart Labels feature allow users to automatically classify all email conversations with certain client. customer or contact.

For example, if you are recruiter and you get an answer from a prospect, then while replying you can automatically label all email conversations with that prospect by just one click:

You can also label all email conversations with domain or company:

So after applying smart labels, all future conversations with that contact or prospect will be automatically labeled.