Chat bots are special accounts that you can chat with as if you were talking to a person, but they provide information to you instead. 

If you add the bot for Google Drive, for example, you will receive chat messages when someone shares a file or folder with you, when someone mentions you in a comment or someone requests access to a file you own. 

To add a bot to Google Chat: 

You will now receive relevant messages from the bot e.g. when files are shared with you.