What is Mattermost / chat?
Mattermost is open source messaging software - see https://www.mattermost.org/

References to Mattermost within this CiviCRM StackExchange site generally refer to CiviCRM's online community chat system that uses the Mattermost software and is one of the various support resources for CiviCRM.

StackExchange uses a Question and Answer format which works well for many things but sometimes an interactive chat format is more appropriate.

You can access it at https://chat.civicrm.org/  Follow the links to create an account.  


It is organised into various public and private channels.  Take note of the description of each channel and post your question in the most relevant one. 
If you're unsure which channel to use, try 'Town Square' or 'User Support'.
Just ask your question. Don't ask if it's ok to ask. 
Choose the most appropriate channel and ask.  Don't post the same question in multiple channels.
It is possible to send "Direct Messages" to other users but don't use them, unless you know the person and you are sure they are the only one that can reply.
Be nice!  It is a community of fellow CiviCRM users who are often very helpful, but do remember no-one is obliged to help you.