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:
Click in the + to the right of the Chat header
Choose Find a Bot
Find the bot you wish to add, then click on the Add link
If given a choice, choose to add a message bot, or to add the bot to a room
Click on Yes to turn on notifications for the bot
You will now receive relevant messages from the bot e.g. when files are shared with you.