Not all terminals have the complete set of modifiers. Terminals that have a <Meta> key allow you to type Meta characters by just holding that key down. To type Meta-a, hold down <META> and press a. On those terminals, the <META> key works like the <SHIFT> key. Such a key is not always labeled <META>, however, as this function is often a special option for a key with some other primary purpose.
If there is no <META> key, you can still type Meta characters using two-character sequences starting with <ESC>. To enter M-a, you could type <ESC> a. To enter C-M-a, you would type ESC C-a. <ESC> is allowed on terminals with Meta keys, too, in case you have formed a habit of using it.
If you are running under X and do not have a <META> key, it is possible to reconfigure some other key to be a <META> key. See Super and Hyper Keys.
Emacs believes the terminal has a <META> key if the variable
meta-flag is non-nil. Normally this is set automatically
according to the termcap entry for your terminal type. However, sometimes
the termcap entry is wrong, and then it is useful to set this variable
yourself. See Variables, for how to do this.
Note: If you are running under the X window system, the setting of
the meta-flag variable is irrelevant.