Unmetered or unlimited bandwidth usually just means that your account is not constrained by bandwidth so it is not something you have to worry about. There is no limit set. This is usually for shared hosting accounts. It is subject to the server resources. For example, if you decided to host a bunch of large files or videos and had a lot of visitors causing your bandwidth usage to reach perhaps 500GB or more in a week or month someone may contact you about it and ask you to upgrade your account to a non shared server. Also some datacenters have their bandwidth set at unmetered or unlimited but the port speed will be 100 Mbps.
Incoming bandwidth is data that is being transferred to your server from another server. Outgoing is data that is being sent from your server to another one. Always read their terms of service.. When it comes to unlimited bandwidth or unlimited disk space, it’s best to think realistically.