What does the “@” symbol mean in reference to lists in Haskell?

Yes, it’s just syntactic sugar, with @ read aloud as “as”. ps@(p:pt) gives you names for

1. the list: ps
2. the list’s head : p
3. the list’s tail: pt

Without the @, you’d have to choose between (1) or (2):(3).

This syntax actually works for any constructor; if you have data Tree a = Tree a [Tree a], then t@(Tree _ kids) gives you access to both the tree and its children.