what ralf is trying to point out is that "indentation" does not mean
inserting blanks before a line, but rather inserting _the right amount_
of
blanks before a line. TeX and LaTeX indentation are similar to
indentation
in computer language source code, in that it is considered practical to
indent code blocks by two blanks.
that means that the "normal" text of a LaTeX file is not indented
(i.e. no blanks are placed before the lines of normal text), an
environment
is indented by two blanks, and each further nested environment is
indented
by another two blanks.
the TAB key in LaTeX mode performs this kind of indentation. that means
that if you hit TAB on a line of normal text, it doesn't remove the
indentation, rather it puts the right amount of blanks in, which just
happens to be zero.