5G system has defined two authentication procedures i.e. EAP based and 5G AKA based in the specification. Though the term "EAP message reliable transport" is bit confusing but it is very simple when it comes to the implementation.
Same as LTE network, 5G system also supports Authentication procedure to fulfill the purpose of mutual authentication between UE and network. AUTHENTICATION REQUEST and AUTHENTICATION RESPONSE messages are used for authentication procedure.
If EAP based authentication is used then these two message of authentication procedure carry EAP messages. The concept of carrying EAP messages within these two high level authentication messages is known as "reliable transport of EAP message".