`M` stands for the Meta key. Commonly represented by the Alt/Option/Esc key on your keyboard.  
`C` stands for the Control key.  
Example -- `C-k` means 'hit `k` while keeping `Control` pressed.'

`M-.`  
Yank the last argument to the previous command.

`M-C-y`  
Yank the first argument to the previous command.

`C-k, C-w, M-d, C-u`  
All these kill text in different ways and save it in the kill-ring. 

`C-y`  
Yank most recent item from the kill-ring.

`M-y`  
Immediately follows a `C-y`. Cycle through the kill-ring.

`!!`  
Substitute the previous command (e.g. `sudo !!`)