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A key sequence (‘key’ for short), is a sequence of one or more input events that is meaningful as a unit. Input events include characters, function keys, and mouse buttons—al the inputs that you can send to the computer. A key sequence gets its meaning from its "binding", which say what command it runs. If a key sequence invokes a command, it is called a complete key. If a key sequence isn’t long enough to invoke a command, it is called a prefix key. Every key sequence is either a complete key or a prefix key.
A prefix key combines with the following input event to to make a longer key sequence. After a prefix key is entered, Emacs waits for further input. The combination might be another prefix key, or a complete key. There is no limit to the length of key sequences, but in practice they are seldom longer than three or four input events. An input event cannot be combined with a complete key.