Bis' writeup is sort of technical, so here's the same in layman terms
M-x is an Emacs key combination. (If the keyboard has "Meta" key, this means meta + x, in modern PC keyboard maps this often means Esc followed by x.)
There you'll get a prompt "M-x", after which you can write all sorts of commands. Tab completion works so you can see all commands.
Some useful commands:
Some commands can get numerical arguments: For that, type C-u argument M-x command. For example, C-u 10 M-x dissociated-press tells Dissociated Press to use 10 character overlap.
You can also make your own elisp functions be launchable this way: Just put (interactive) as the first command in the function.