Looks vaguely reminiscent of ML/Ocaml/Fsharp typing where 'a is used to indicate an unbound type variable. E.g. a list of integers would be int list, whereas the polymorphic empty list [] has a type 'a list; a simple tree datatype with nodes capable of holding elements of any arbitrary type could be declared like:

type 'a tree =
  | Leaf
  | Node of 'a tree * 'a * 'a tree
[download]

Given that my guess is it's attempting to say that whatever can be a reference to any arbitrary type of value.

Edit: Ocaml data type tutorial for more examples and explanation.

Morning thought: Actually their tree example looks weird because (upon further reflection) I'd have expected the leaf constructor to be Leaf 'a (to hold a value of 'a in it, vs a node which has a tuple of the left and right 'a tree with the node's item's 'a value). Then again I've just been intermittently reading Real World Ocaml off and on so . . . .