M-x command means type M-x, then type the name of the command, then type <RET>. (See Basic keys, if you're not sure what M-x and <RET> mean.)
M-x (by default) invokes the command
execute-extended-command. This command allows you to
run any Emacs command if you can remember the command's name. If
you can't remember the command's name, you can type <TAB>
and <SPC> for completion, <?> for a list of
possibilities, and M-p and M-n (or up-arrow
and down-arrow) to see previous commands entered. An Emacs
command is an interactive Emacs
function.
Your system
administrator may have bound other key sequences to invoke
execute-extended-command. A function key labeled
Do is a good candidate for this, on keyboards that
have such a key.
If you need to run non-interactive Emacs functions, see Evaluating Emacs Lisp code.