I suppose Marx’s discussion on alienation is less about some ultimate meaning of life (which many people have already talked about here) and more about how people find meaning in their work, their humanity and the wider world, and how the way our society currently works alienates us from those things and from finding our own meaning, instead pushing us to act like cogs in a pointless machine. For example, if someone’s waking hours are mostly spent making useless things for people they’ll never meet, or denying people medical coverage, they’re going to develop a very different sense of their meaningfulness than someone who builds houses in their neighbourhood, or who grows or prepares food for their family and friends. Both are labouring in order to survive, but the latter can see much clearer how their actions matter.

(I’m probably butchering it, this isn’t a theory I know much about, so check to see if someone else has corrected me)