To me it means that a person think or act in a manner that are new but do not conform to present norms or beliefs in society. The expression "his time" really should be "our time" but it implies that every person has, in some sense, a decided time and play to act on in society. "Ahead" means that he plays a part on the stage before it should be played.
Yes Einstein probably had a clue of what he started to understand before he could express it in the formula.(before “his time”) To define and prove something scientifically is both a way to test its truth but also a language of communicating it to others. This is especially important in a paradigm shift. Science is to some extent just conformity between scientists. He was then ahead of his time in the meaning that he still couldnt prove it by a formula. This must have been important to him otherwise he would not put the effort to it.
Yes, conformity in a more or less totalitarian way was an absolute norm both in society and science, during the era of Einstein’s work with the relativity theory. That might have suppressed many other scientists to think out of the box. After his breakthrough science came to a complete turn into alternative thinking by Bohr and others.
Much more can be said but I leave it to others.
Thank You for your question!