What does a comma in the guard syntax do?

Guards are described in Haskell 2010 section 3.13, Case Expressions
(that section is about case expressions, not top-level declarations, but presumably the semantics are the same):

guards  →  | guard1, …, guardn      (n ≥ 1)
guard   →  pat <- infixexp         (pattern guard)
        |  let decls               (local declaration)
        |  infixexp                (boolean guard)

For each guarded expression, the comma-separated guards are tried sequentially from left to right. If all of them succeed, then the corresponding expression is evaluated in the environment extended with the bindings introduced by the guards. That is, the bindings that are introduced by a guard (either by using a let clause or a pattern guard) are in scope in the following guards and the corresponding expression. If any of the guards fail, then this guarded expression fails and the next guarded expression is tried.

In the simple case, the comma serves a role similar to Boolean and. But the comma is more powerful in that each guard can introduce new bindings that are used by the subsequent guards (proceeding from left to right).

Commas in guards are uncommon enough (in my experience, at least) that I’d describe this feature as Haskell trivia — not at all necessary to writing (or, for the most part, reading) Haskell. I suspect that Haskell Programming from first principles omits it for that reason.