What is a move that "is recorded in such a way that its true significance is impossible to establish"? According to FIDE rule E.8 on temporary adjournment (my emphasis),


  Except in the cases mentioned in Articles 5, 6.9, 9.6 and 9.7, the game is lost by a player whose recording of his sealed move:
  
  a. is ambiguous, or
  
  b. is recorded in such a way that its true significance is impossible to establish, or
  
  c. is illegal.


The items a. and c. above are pretty clear. An illegal move is, well, illegal (e.g. moving a pawn backwards, moving one's king into check), and an ambiguous move is one that does not fully indicate the piece to be moved and its destination, such as "I move my bishop back", "castling", or "I'm gonna queen a pawn".

What is the meaning of clause b. above? My instinct says that it refers to a move that is unambiguous on its face, but that is not sufficiently identified as the player's sealed move, but it is unclear how that would also not qualify under E.8.a as an ambiguous move, and I found an older version of the rules which states the following (emphasis mine):


  10.16
  
  At the resumption, the game is lost by a player whose recording of his sealed move:
  
  (a) is ambiguous; or 
  
  (b) would result in a false move the true significance of which is impossible to establish; or 
  
  (c) would result in an illegal move.


This version seems to emphasize that this rule is intended to prevent "false" moves as opposed to ones that are merely ambiguous or illegal. E.g. the player's sealed move might be perfectly unambiguous and 100% legal, but it might still be unacceptable because it is somehow "false". It also implies that a move that is "false", but not ambiguous or illegal, may be permitted if its "true significance" can be established, but that seems impossibly unclear.

What kind of specific misconduct or poor play does this rule forbid?
an ambiguous move is one that does not fully indicate the piece to be
  moved and its destination, such as "I move my bishop back",
  "castling", or "I'm gonna queen a pawn".


No. You obviously haven't reached "Recording of the Moves" and clarifications given in the glossary. A move, to be a move, has to be written in algebraic notation. None of the expressions you have written qualify as moves let alone ambiguous moves.

If the player had rooks on a7 and c7 and he wrote Rb7 then that would be ambiguous.


  What is the meaning of clause b. above?


If the person trying to read the move can't work out what the move is because part or all of it is illegible then that would qualify. In general this a catch-all clause that basically says you can't work out what the move is for some reason other than ambiguity. Complete absence would also do. i.e. the player seals a blank sheet of paper.