I think it may have been in the context of neural networks or something...
Ah ok, you don't have a specific need? Scale-free networks tend to occur in social networks (probably in Nostr too), so they're important to know about if you want to do network analysis on such a network
What does “scale-free” mean
In the context of a network, "scale-free" means that the degree distribution follows a power law. Degree distribution is math speak for "how many followers does a typical npub have", and a power law is a distribution with so-called fat tails, meaning that some npubs have a huge number of followers, BUT many npubs (the tail) have non-negligible numbers of followers to.