The reasoning for the symbols was to give the band some anonymity after criticism from the music press, although each symbol represents each member of the band and has a meaning specific to that individual.
From left to right the symbols represent; Jimmy Page, John Paul Jones, John Bonham and Robert Plant
Page's symbol is rumoured to have a variety of meanings: anything from the occult to representing Saturn. It also bares a resemblance to the alchemical for mercury.
Jones' symbol represents confidence and competence and was taken from a book of runes.
Bonham's represents man, woman and child but when reversed the symbol represents Ballantine beer (fitting as Bonham was a notorious boozer). Apparently Bonham chose it simply because he liked it but it was Plant who later detailed its meaning. His symbol was taken from the same book as Jones'.
Plant's represents justice and fairness, the feather is that of Ma'at, the Egyptian goddess of justice and fairness, and it is also the emblem of a writer.
This use of symbols could be seen as an unintentional method of branding the members of the band.
As for me, my favourite is John Bonham's symbol, possibly due to its symmetry and use of circles followed by Jones'.