The default welcome files are the usual index.cfm, index.htm, index.html, etc but you can override this with the welcomeFiles setting in your server.json by providing a comma-delimited list of files that you would like CommandBox to look for when a user hits a directory on a running server.
server set web.welcomeFiles='go.cfm,main.cfm,index.html'
This setting is a complete override of the defaults, so you need to specify the full list.
By default, a CommandBox server will not show the contents of a directory that doesn't have an index file. You can enable directory browsing for a single server with
server set web.directoryBrowsing=true
And you can enable it for all servers by default with
config set server.defaults.web.directoryBrowsing=true