What did Maz Kanata give to Chewie in the end of the Episode IX? Have we seen this (golden?) object before? What makes it significant?
As TheLethalCarrot said, it's one of the Alliance medals given to Luke and Han in A New Hope.
My interpretation of its significance had been that it's a response to Chewie not getting a medal after the Battle of Yavin, which some fans have seen as unfair (since he didn't do any less than Han), and which has had varying explanations and retcons over the years.
It's not clear whether Chewie getting the medal is an in-universe acknowledgement of his bravery during the Battle of Exegol, or if it really was just Han's and Maz Kanata had it lying around. He apparently did get a medal after Yavin according to Disney canon, so it would have to be either a retcon or a second medal, but he waves it around triumphantly after receiving it, so it would be odd if that was his reaction to receiving something of Han's. A third possibility is that it's just another medal of the same type, and has no deeper significance in-universe.