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.