Ok, I give it a try.
the guest account is used to give access to a database without explicitely creating a databaseuser for a login. The best example is the mastter database. Most SQL logins are not created as a user in master. But for login and certain other actions the login must access sytem tables and procedures in master. Because the guest user is enabled in master, every login even though it is not an explicit usere in master, can access these system tables. The guest user has the permissions of the public role.
By default the guest user is enabled only in master, msdb and tempdb. In model and all user databases the guest user is disabled.