1.1 What do these mean: C-h, C-M-a, RET, ESC a, etc.?

• C-x: press the x key while holding down the Control key
• M-x: press the x key while holding down the Meta key (if your computer doesn’t have a Meta key, see What if I don’t have a Meta key?)
• M-C-x: press the x key while holding down both Control and Meta
• C-M-x: a synonym for the above
• LFD: Linefeed or Newline; same as C-j
• RET: Return, sometimes marked Enter; same as C-m
• DEL: Delete, usually not the same as Backspace; same as C-? (see Why does the Backspace key invoke help?, if deleting invokes Emacs help)
• ESC: Escape; same as C-[
• TAB: Tab; same as C-i
• SPC: Space bar

Key sequences longer than one key (and some single-key sequences) are written inside quotes or on lines by themselves, like this:

  M-x frobnicate-while-foo RET

Any real spaces in such a key sequence should be ignored; only SPC really means press the space key.

The ASCII code sent by C-x (except for C-?) is the value that would be sent by pressing just x minus 96 (or 64 for upper-case X) and will be from 0 to 31. On Unix and GNU/Linux terminals, the ASCII code sent by M-x is the sum of 128 and the ASCII code that would be sent by pressing just x. Essentially, Control turns off bits 5 and 6 and Meta turns on bit 7¹.

C-? (aka DEL) is ASCII code 127. It is a misnomer to call C-? a “control” key, since 127 has both bits 5 and 6 turned ON. Also, on very few keyboards does C-? generate ASCII code 127.

See Keys in The GNU Emacs Manual.