What do people mean when they talk about forward secrecy in ciphers?
What does 'forward secrecy' mean in the context of hash based signatures?


Well, what that term means is that we have a hash based signature scheme with an evolving key, that is, the private key is constantly being updated, and on a state compromise (at state $N$), you cannot get the private keys used prior to state $N$.

I personally don't see that as a good use of the term (it's feels like 'perfect reverse security' would be more apt terminology, as it's discussing the security of the system prior to compromise).  However, that's what people mean by that terminology.