What does the prime symbol mean in organic nomenclature (e.g 2')?
Primes, double primes, triple primes, etc., are added to differentiate between the same locant in different parts of the structure. According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), they are used


to differentiate the nitrogen atoms of hydrazides, imidamides (amidines), amidrazones, and hydrazidines;


  N'-acetyl-N'-ethyl-N-methylpropanehydrazide

with superscripted numerical locants indicating the position of the suffix on the parent structure, to differentiate the nitrogen atoms in other cases, especially when symmetry conditions are not fulfilled;*


  N3-ethyl-N'3-methylhexane-3,3-diamine

in multiplicative nomenclature to denote multiplied units and modify locants accordingy;


  2,2',2''-nitrilotri(ethan-1-ol)

in spiro-fused compounds, to denote positions in polycyclic systems


  7,7'-spirobi[bicyclo[4.1.0]heptane]

in ring assemblies, to number identical ring components;


  1,1'-biphenyl  

in fusion nomenclature, to identify first and higher attached components, identical attached components, and multiparent names;


  difuro[3,2-b:2',3'-e]pyridine

in fullerenes ortho-fused to organic ring or ring systems, to identify positions in the nonfullerene component;


  3'H-cyclopropa[1,9](C60-Ih)[5,6]fullerene

in natural product nomenclature, to identify positions in ring(s) fused to a fundamental parent hydride;


  naphtho[2',1':2,3]-5α-androstane



The example that is given in the question is a ring assembly. The numbering of locants corresponds to the numbering of 1,1'-biphenyl, where primes are used to differentiate between the same locant in different phenyl groups. Therefore, the correct systematic name is 4,4',5,5',6,6'-hexahydroxy-[1,1'-biphenyl]-2,2'-dicarboxylic acid.



* Note that, according to current IUPAC recommendations, primes are no longer used to differentiate the nitrogen atoms of symmetrical geminal diamines, diimines, diamides, di(imidamides), (diamidines), di(hydrazonamides), di(amidrazones), di(hydrazonohydrazides), imidohydrazides, and poylimidopolycarbonic and inorganic oxoacids.