That's for a reason: You will find a lot of speculation but the plain answer is: we do not know. Now you will say "But ..." and I will repeat "We do not know".
There is, for instance, this burial mound in Hochdorf, Germany, and its "inhabitant" is sometimes referred to as a "Fürst" = Lord because of the extraordinary riches that were buried together with him, and the fact that the grave was not robbed after it had been laid and was mostly intact.
But the findings are by no means a hint to his importance or social role during his life. We don't know the habits of those people in life, and burial mounds weren't rare at that time.
There are a lot of interesting places from the time between late bronze age (e.g. Urnfield) and La Tene culture all over Europe, together with reconstructions of the material legacy and residues (e.g. Heuneburg) and of course the eponymous Hallstatt in Austria, but no hints to social stratification that would make sense in our eyes.