What's the meaning of strict version in haskell?

Question

Follow <Real World Haskell> , it is said foldl' are strict version of foldl.

But it's hard for me to understand , what does strict mean??

foldl f z0 xs0 = lgo z0 xs0
         where
            lgo z []     =  z
            lgo z (x:xs) = lgo (f z x) xs


foldl' f z0 xs0 = lgo z0 xs0
    where lgo z []     = z
          lgo z (x:xs) = let z' = f z x in z' `seq` lgo z' xs

Answer 1

foldl and (the strict) foldl' are close to semantically equivalent. The difference is in performance, especially when you are transversing a large list. The laziness has an overhead of building a thunk and foldl' is the more efficient way to arrive at that result because it doesn't build a huge thunk.

There is a really good article explaining this in detail on Haskell Wiki

Strict functions works like functions in C or other languages in that their arguments are generally eagerly evaluated.