What is exactly an indexed functor in Haskell and what are its usages?

When studying functors in Haskell I came up with Functor.Indexed type of functor. This functor defines an operation called imap. I didn't understood its definition and imap signature: imap :: (a -> b) -> f j k a -> f j k b. I tried to find it s formal definition and found only this: http://ncatlab.org/nlab/show/indexed+functor . But it really didn't help me at all. So can someone clarify in more simple words this kind of functor and in what cases should I use it? Thanks.