Sorting a list of lists according to length of sublists

let rec insert cmp e = function   | [] -> [e]   | h :: t as l ->     if cmp e h <= 0 then e :: l     else h :: insert cmp e tlet rec sort cmp = function  | [] -> []  | h :: t -> insert cmp h (sort cmp t)let length_sort lists =  let lists = List.map (fun list -> List.length list, list) lists in  let lists = sort (fun a b -> compare (fst a) (fst b)) lists in  List.map snd lists;;

I want to know how work this fun a b -> compare (fst a) (fst b),compare return 1 or -1, and what will give as variable to sort?and List.map snd lists will remove the first element of every list in the lists?