What does the '@' symbol and hostname mean in the context of 'Access Denied' errors?

In MySQL/MariaDB, user accounts are defined as 'username'@'hostname'. The '@' separates the username from the hostname from which the user is allowed to connect. For example, 'myuser'@'localhost' means 'myuser' can only connect from the same machine where the database server is running. 'myuser'@'%' means 'myuser' can connect from any host.