I've encountered some man pages which say 'press m-tab to do '. I figured C-tab means ctrl+tab, but can't understand what does m-tab mean. Anybody knows what is it?
M is short for Meta. In the 1970s and 1980s, keyboards on machines running Unix systems often had three modifier keys: Shift, Control and Meta. For this reason, most applications use these three names as modifiers. Then in the 1990s people used more an more PC keyboards, which had no key labeled Meta but had a key labeled Alt, so the key labeled Alt was configured to send the
Meta modifier expected by applications. Thus, when an application has key bindings with the
Meta modifier, use the Alt key.
On classic-era keyboards, Shift and Control were ubiquitous, but Meta wasn't. Furthermore, Shift and Control had a well-defined effect on most ASCII characters, but Meta didn't. To indicate Meta, some terminals set the upper bit of the characters, which wasn't used for anything else, but that interfered with 8-bit character sets used in countries with an alphabetic script that went beyond plain Latin letters, either with a different alphabet such as Cyrillic or with diacritics. So another encoding was often used: Meta+key was encoded as Escape followed by key. With applications running in a text terminal (text mode console or terminal emulator under a graphical interface), you can press Escape followed by the key to trigger a Meta+key shortcut.
M-tab, it's likely that you'll need to press Esc Tab, because most window manager intercept Meta+Tab (same as Alt+Tab, since Alt is mapped to Meta) as a shortcut to switch between windows. A few terminals also send the same key sequence for Shift+Tab, but it depends on how the terminal emulator is configured.