These terminals use a sequence of characters to represent each function key. But that is invisible to the Emacs user, because the keyboard input routines recognize these special sequences and convert them to function key events before any other part of Emacs gets to see them. But Emacs also has commands that take two or more events to invoke. If a sequence of events is enough to invoke a command, it is a complete key . If it isn't long enough to be complete, we call it a prefix key .
Every key sequence is either a complete key or a prefix key.
Most single characters constitute complete keys in the standard Emacs command bindings. A few of them are prefix keys. A prefix key combines with the following input event to make a longer key sequence, which may itself be complete or a prefix. There's no limit to the length of a key sequence, but in practice people rarely use sequences longer than four events. By contrast, you can't add more events onto a complete key.