I think that the title is completely clear, but here's an expansion:
I was just reading about Mass-luminosity relation that says massive stars are more luminous than tiny ones. Well, let's talk about main-sequence stars for now. This relation becomes interesting when it's mixed with the Hertzsprung-Russel diagram that says young stars are more luminous than older ones (Originally says that hotter stars are more luminous and we know that hotter stars are younger).
So mixing these two relations produces some kind of Mass-age relation which says young stars are more massive, that is a star loses its mass over time, right? (Please tell me if I'm going wrong)
If this Mass-age relation is right, how do stars lose this extra mass during their evolution? And where does this stray matter go?