Could be a macro or a perform statement. In it's simplest form it is a display of a specific message number from message class.
In this example below, it will display a success message, "All buffers reset" which is message number 012 from message class 00.
REPORT zdemo82 MESSAGE-ID 00.
DEFINE message_simple.
message s012.
END-OF-DEFINITION.
START-OF-SELECTION.
message_simple.
