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.