scores for chess moves?

How does one interpert a engine rating a particular move made in an analysis?  For example:  0.4 is good?, 1.04 is really good?, 0.01 is still good just not that good?  negative numbers are always bad?  I've seen numbers as high as three and as low as negative three.  Any insight would be great, thx joe

0.01 is practically even, 0.4 is still pretty even as well given White's advantage. Basically, the higher positive score, the more the position is judged to favour White, whereas the higher negative, the more Black is favoured. And its in terms of points, all aspects such as position are somehow translated into the equivalence of points. So 1 means White is the equivalent of up a pawn.

It all began with Claude Shannon's paper on the subject back in 1950:

http://en.wikipedia.org/wiki/Claude_Shannon#Shannon.27s_computer_chess_program

This one is also very good, as are some of the references sited:

http://en.wikipedia.org/wiki/Chess_piece_relative_value

@Niven42, thanks for your comments, very helpful.

It took you a bit less than 14 years to read it, good job.